Burnout is stress to the point that performance and motivation to continue doing action drop a lot.
Having tons of flashcard reviews is a common cause of that because it may make the person feel nihilistic since it's so repetitive, cognitively draining, and boring. Sleep can be a good counter... but the education system pretty much prevents that due to deadlines, and stress can impact sleep itself!
Is it a blanket for everything: exhaustion, external pressure, stress? Honestly, what even is it, to the point that it is thrown about everywhere, yet I KNOW IT IS incurable, and I know why, and all men live to suffer in vain.