It means that the JUnit libraries are findable by the project, but not part of the project.
You need to be able to find the JUnit libraries when you run jUnit tests, but you should not be bundling jUnit or your test classes into your production build, so both test source and test libraries should be located outside the main project build tree. For example, in Maven builds, production code goes under /src/main/java, test code goes under /src/test/java.
The other characteristic of external libraries is that since they aren't made part of the project source, you can share the same copy with multiple projects instead of having a distinct copy of the library for each project using it.
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What's that smell? I think this tiny ad may have stepped in something.