Saketh loved teaching because he could tell right away when students “got it”—their faces would light up, and he’d know he had done his job well and them learn.

This is an example of an intrinsic reward.
Intrinsic rewards are internal - they refer to feelings of accomplishment, of success, of pride, and are intangible, meaning that you cannot touch them. On the other hand, extrinsic rewards are actual objects that you receive as a reward for something you did.