The subtasks are tasks associated with the activities of the project.
Subtasks are best used in instances where a project moves at different levels. For example, "Onboard New Client" is one large task, but there are actually several moving pieces and teams just within that one task. It's not uncommon for the original task at hand to be modified 
You can create Tasks and sub-tasks in Project Management and assign them to the users.

Click here to know more about creating a Task Assignment

To create a sub-task,
  • You see a list of Projects and Task Assignments in the Gantt chart
  • To add a subtask to a Task Assignment,
  • Click the + button that is at the end of the task assignment row
  • Add the subtask details and save it
  • You can create multiple sub-tasks to the Tasks Assignment in the Project Management
  • To view the subtasks, click the small drop-down icon of a task , and you see the sub-tasks as in the screenshot below (marked in green).