It is a test from the National League of Nurses that is standardized, and given to all students at the same level of training, nationwide. It evaluates not only how you are retaining what you have been taught, including critical thinking, but how well you are BEING taught. The scores of the class, as a whole, reflect how well the instructors are doing their jobs.
The test is not one that you can study for-it is kind of like semester tests in high school. A little class review of topics covered during the semester might help, as well as reviewing your notes. However, it is a broad-based test, and in-depth study is not practical. It is one of those "either you learned the material, or you didn't," kind of tests.
Good luck to you, and keep studying!