A gold star is one of these (sometimes sold in sheets with more than just gold stars). To give one, you'd take one off the sheet. To get one, well, it depends on what the other person wants from you. Mostly, though, it's for doing well at something in elementary school.

This reward is so memorable that people give and get virtual gold stars that function as a metaphor for it. For example, you might see on a Reddit thread, "hey, that was a great {comment | answer | ...}! here's a gold star!".