According to JMeter Tear Down Threadgroup help ,
A special type of ThreadGroup that can be used to perform an Action after testing. The behavior of these flows is exactly like a regular element of a group of elements. The difference is that these types of threads are executed after the test has completed its regular Topic Groups.
In simple words: This group of threads is used to perform actions that are performed / necessary after the tests are completed. that is, after testing. This thread group will be executed after the completion of the normal thread group.
This thread group can be used in scripts that need to perform certain actions after the test runs. Simple examples could be sending reset requests to the server, some warm-up requests for shutdowns, or some configuration changes or client-side execution.
Ex. After completing my test If I want to delete the user that I created only for the test. That way I can send deleteuser request to threading chain. Another example would be: If, after the test, I want some reporting material to be displayed in the response data, then you can add a sample Java code (beanshell processor) to the stall chain of threads and perform the operation.
Keep in mind that the operation can be any, server request or DB request or sample Java code, etc. There is a condition, this should be done after the test.