As already mentioned, "burning out" is losing the energy or desire to study (or even do basic life activities because you just cbf). Think of the imagery of a fire burning out. It's burning, then it fizzles, and eventually there's no more flame.
I'd liken it to not recovering properly from consistently intense exercise. For example, if you get up every day and do a series of sprints, then head to the gym and lift really heavy weights, but you don't taken in enough carbohydrates/water/protein/sleep, after a while your performance will significantly drop because your nervous system just hasn't recovered. Your CNS has a 'cloud' over it and your peripheral nervous system just doesn't have the fuel to keep exercising. Burnout is a similar feeling to this, where it feels like your nervous system has just given up on you.
As far as exercise goes, this happens when you don't recover properly. Sleeping, eating etc.
I'd say it's similar with study. It's not necessarily study itself that causes burnout, but demented habits that arise from studying. Such as, shitty sleeping habits, shitty eating habits, no leisure time because you're constantly too anxious about studying to relax. I mean, how is it that some people play rugby/soccer 3 nights a week, go to a party every so often and still manage to get 90+ ATARs? You'd think these people would "burnout" from all the energy they're expending, but it's not about direct time investment, rather about the way you invest your time. If you invest your time constantly studying in unenjoyable ways and not eating/sleeping/seeing friends/enjoying your life, your motivation will obviously be shot to shit real quick. Just make sure you structure your study in a way that allows you to enjoy life and not be consumed by the HSC, and you won't burnout. If you feel like you are just simply incapable of studying, then don't force yourself to. Take a week off and just come back with a vengeance later.
Edit: Lol first post, just found this community today. Wish I found it in high school lol!