JUnit is a unit testing framework for Java applications, including Android apps. It allows developers to thoroughly test code to ensure it works as intended. Some key advantages of JUnit include that it is written in Java so no new language needs to be learned, tests can be run within Eclipse without extra downloads, and it is free to use. Maintaining JUnit tests and code is also relatively easy. The downsides are that developers must write the tests themselves, which takes time, and tests are limited to available devices. The document then provides tutorials and examples for setting up and running basic JUnit tests on an Android project.