What is purpose of atlassian-plugin.xml in test's resource?

I am a new guy of making jira - plugin development…
So, everything related to jira-plugin development is new…

In the project, there is another atlassian-plugin.xml…
I know that it is for support to test some kinds of unit… but I don’t know how to use it??
so… when I make a junit module for testing something, do I need to write some code in it??

Are there any kinds of materials for understanding it??