AQF is Australian Qualifications Framework. The AQF is the national policy for regulated qualifications in Australian education and training.
AQF levels are an indication of the relative complexity from AQF 1 to 10.
Level 1: Certificate I
Level 2: Certificate II
Level 3: Certificate III
Level 4: Certificate IV
Level 5: Diploma
Level 6: Advanced Diploma, Associate Degree
Level 7: Bachelor Degree
Level 8: Bachelor Honors, Graduate Certificate, Graduate Diploma
Level 9: Masters Degree
Level 10: Doctoral Degree
Please check it out at AQF website for more details.
For the student visa (500), you are not supposed to changed your course to lower AQF level. If you do so then you need to apply a new student visa.