technically its the "aggregate of your scaled marks" (which is fancy statistics, but basically means that they will sum your scaled marks) and there is no such thing as atar contribution, but in effect it's just a statistic to tell you what scaled marks you'd need in every subject to get a particular atar. for example in 2023, you needed an aggregate of 450 to achieve an atar of 99, meaning that, your scaled marks needed to sum to 450. hence, the scaled mark /50 you'd need to get a "99 atar contribution" would be 450/10 = 45, assuming that they're using scaled marks per unit. for example in english advanced, a hsc mark of 47.5/50 was scaled to a 46.7/50, which means that mark would have an "atar contribution" of roughly 99.2 (as a guess). i wouldn't worry too much about it, all this tells you is that you need consistently good marks in all your subjects to achieve a high atar. for example even if you got a 50 scaled (basically 100% raw mark) in 6/10 of your units, and then only got 25 scaled in the other 4/10 units (probably a band 3-4 in most well scaling units) you would have 400 aggregate, which corresponds to a 90 atar (still pretty good lol). hence even if you're godly at 3/5 of your units, your other units will drag you down (and that's in the optimal situation where you max out your scaled mark in your best units). so just try to focus on doing at least moderately well in your worst subjects, as well as obviously maxing your marks in your best ones
(fyi basically everything is in here
https://www.uac.edu.au/assets/documents/scaling-reports/scaling-report-2023-nsw-hsc.pdf if you really want to get into the details)