It is simply the course code for each course you've taken. If you took Zoology 460 then you would put "Zoo460". If you took Math 110, then put "MAT110".
Many schools use four-digit course codes (Zoo 4610 instead of Zoo 460), but that doesn't matter. You can put anything you want in there, as long as adcoms can easily match your AADSAS courses to ones listed on your actual transcript.