JIRA issue is a standard and standalone issue. But JIRA facilitates broken down into logical sub-issues. And It can be achieved in multiple ways.

Task- A task is represent the work needs to done by a day or in a small piece of time.

Sub-task-  Subtask is a piece of work that is required to complete a task.

Example 1
We have a ticket related to a functionality Account Object creation in Salesforce, but there is a multiple things needs to be done:
-  Create fields
-  Create Flexi Page
-  Put Validation for mandatory fields
-  Link of contacts
-  Addresses

So Their is a ticket for Account Object creation:

1. Task - for fields creation – assigned to resource 1
     - Subtask for validation creation for all required fields – assigned to resource1
2. Task - Link of Contact – assigned to resource 2
     - Subtask to sync fields b/w them – assigned to resource 2
3. Task - Link of Address – assigned to resource 3
     - Subtask to create type of addresses and syncing of fields – assigned to resource 3

One more example we can put related to QA -

Example 2
We have a story and for that we have a Task to achieve Testing or Test Documentation, Test documentation by itself is an activity that might take a week to finish.  Say, it involves the following aspects:

Test plan documentation which takes 2 days. Test case documentation – 2 days, Test plan review – ½ day and Test case review – 1 day.  Also, assume that there are 2 resources.

In this case, we can create a JIRA issue of the typing task for “Test documentation” and have the following four subtasks under it:

1. Test plan documentation – assigned to resource 1
2. Test case documentation – assigned to resource 2
3. Test plan review- assigned to resource 2
4. Test case review- assigned to resource 1

By doing so, it is easier to have a better insight into the progress task wise and resource-wise by breaking a sizeable parent task into sub-tasks.

Jira have multiple issues to track all the work of project. Task and Subtask one of them. Task is an independent issue type while a Sub-task needs a parent issue.

Task - A task is a standalone issue type in Jira. Tasks allowing you to quickly describe and divided the work that needs to be done by your team – all in a single Story.

For example- Story to develop a new feature for an existing application. To get that feature released you could have individual pieces of the development, testing, and release as tasks. 

Sub-task - When task is too big and complex then the task can be broken down into logical sub-issues known as ‘Sub-tasks’ in Jira.

Example 1
We have a story related to a data migration of Object from different org in Salesforce 

We further divide into below task 

-  Layout of object
-  Create fields of object
-  Field permission of fields

Further divided task into sub-task

  Layout of object - Subtask for QA Test Scenario & Test Case Creation and Test Execution
 

In my opinion, Jira is used to track many different types of issues. One of those types is 'Task'.
A Task Can be defined as a small, distinct, independent piece of work.
Subtasks are child issues that are created under a parent task.
 

For instance, let's consider a scenario where a team is working on a user story that needs configuring backend settings. Prior to commencing work on the user story, a Task will be created to handle the backend configurations.
 

Task: Configure Backend Settings

Within this configuration task, several specific aspects need attention. For example, toggling a button, setting predefined tax rates, specifying the country name, and configuring the correct server and region. All these can be further categorized a sub-tasks.
 

Subtask 1: Toggle the button
Subtask 2: Set predefined tax rates
Subtask 3: Specify the country name
Subtask 4: Configure the correct server and region
 

By utilizing subtasks, the team can efficiently address each distinct aspect of the main task, contributing to the successful implementation of the user story.

From my own experience, we have (in a company I’m currently working at) agreed that we’ll use tasks to describe and track the development of a larger feature while we use subtasks for tracking smaller chunks of said feature. 

Example:

Task: 

Create a new menu item and related form that’ll be used to track business trips

Subtasks:

1. Create the menu item that should be only visible to admins 
2. Create the form that has the following characteristics…
3. Develop a method that will do XYZ
4. ...
5. …

We found that structuring the job like this gave us the best overview of the feature being developed, while it helps us testers to tackle one part of the feature per time. Of course, task isn’t considered complete until all the subtasks are finished and until the final testing cycle is successfully completed 

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Jira: One of the most effective and widely used tools in the testing industry is Jira. It enables us to prioritize, plan, and keep track of the numerous issues.

One of its features is task and sub-task.

Task: In plain English, it refers to the work that needs to be completed. Jira uses the same meaning.

Here's an illustration:

Let's say a Jira story with the title "Mobile App - Trade In flow (Apple Watch, iPhone, Mac and iPad)" has been added.

After reading the user story, it becomes clear what things need to be done by the appropriate parties (such as the developer and QA) in order to finish it. The word "things" in this context denotes "task".

Suppose that after reading the story, the following tasks emerge:

Task 1: UI design.
Task 2: API creation.
Task 3: Writing Code.
Task 4: Testing.

Therefore, we may say that a task is any action needed to fulfil a user story.

Sub-task: A sub-task is a subset of a task, so in order to finish the task, we must first finish every sub-task in the task.

In continuation with above example.

The tester will divide each task/assignment (logical one) into smaller ones when conducting tests. 

Suppose following sub-task emerges:

Sub-task 1: Test the trade in flow for iPhone.
Sub-task 2: Test the trade in flow for Mac.
Sub-task 3: Test the trade in flow for Apple Watch.
Sub-task 4: Test the trade in flow for iPad.

Hence, we can say that the task is automatically completed once all of the sub-tasks have been completed.

Likewise, once all the tasks are finished, automatically the story gets completed.

Thus, in my opinion, the work that must be done in Jira to finish a story is referred to as a task, and the work that must be done to finish a task is referred to as a sub-task.

All Answers are good and it was a close fight between Jayesh and Shelja. Both explained with examples, but Jayesh’s answer was more near reality and Shelja’s answer was more general.

Due to Jayesh’s answer is more real, I chose his as best answer.