Masking is someone suppressing their natural autistic behaviors. It's not even necessarily conscious but it takes a large toll on mental energy. Besides the energy required for the masking itself there's the compounding effect of setting unrealistic expectations in others. The better someone is at masking the more is expected of them which creates an even larger drain on mental energy.

>Masking is someone suppressing their natural autistic behaviors.

Well, masking is done by everybody, not just autists, but in this game it's used to refer to that specifically. Arguably autists have to do it more, or feel the exhaustion from it more strongly.