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.
Some people, when well-known sources tell them that fire will burn them, don't put their hands in the fire.
Some people, being skeptical, will put their hands in the fire, get burned, and learn not to put their hands in the fire.
And some people, believing that they know better than well-known sources, will claim it's a lie, put their hands in the fire, and continue to scream it's a lie even as their hands burn down to charred stumps.
You guys haven't done this much, have ya? I suggest you study this tiny ad: