What is a thread group in JMeter?

A group of virtual users is represented in the JMeter application by a Thread Group.
A thread group is the base element (parent) needed in every test plan.
It is actually an assembly of threads executing the same scenario.

Thread Group – This can be used for simple load test scenarios, where you set a predefined number of users and ramp-up period and you start running your script in times. We can use Thread Group for Stress Testing also by increasing the concurrent users. This can be used for Endurance Testing by maintaining the load for predefined period of time.