Validators run software that checks every transaction against the protocol rules, votes on which block is correct, and occasionally gets to propose one. They earn a share of issuance and fees for doing it honestly.
Running one yourself requires a minimum bond, reliable uptime, and correct key handling. Most holders instead delegate to an operator or use liquid-staking, which trades some decentralisation and counterparty risk for convenience.
Example: at a 32-coin minimum and a 3.5% annual reward rate, a validator earns about 1.12 coins a year. Being offline is a slow bleed of missed attestations; double-signing is a fast loss through slashing, and operator fees of 5-15% come off the top.
Related: proof-of-stake, slashing, staking, liquid-staking