NIM stands for "Nomor Induk Mahasiswa". It is essentially a student identification number. It is composed of four parts separated by slashes. The first set of numbers is the student identification number within the Faculty of Medicine. The second set, a two digit number, identifies the faculty (in this case: the faculty of medicine). The second set of two digit numbers is the student's batch year. The final set of numbers is UGM's student identifier.
Often, students only write the first set of numbers on their lab reports and other assignments. Memorizing just the first set is sufficient.