Those are different levels of the JLPT language exam, with N5 being the beginner and N1 - advanced. The vocabulary/grammar schedules on your dashboard correspond to those levels.
You could argue that the N also stands for new, when the levels changed from 1級~4級 to N1~N5 to add an in-between step because 3級 to 2級 was this impossible step-up in difficulty, almost like going from N4 to N2. They had to differentiate to make it clear in English which was old (just a number) or new (N+number).
it helped a lot