Validator

A validator does on a proof-of-stake chain what a miner does on a proof-of-work one: proposes new blocks and checks other people's. Instead of burning electricity, a validator puts up coins as a deposit, called a stake.

Behave and you earn rewards. Cheat, or sometimes just go offline, and part of your stake can be destroyed, a penalty called slashing. Running a validator takes technical skill and, on some chains, a large stake, so many holders instead delegate their coins to a validator and share the rewards.