A validator is the proof-of-stake equivalent of a miner. Validators are network participants who have staked tokens and run nodes that propose new blocks (when randomly selected) and attest to blocks proposed by others. Their attestations are cryptographically signed and aggregated into the chain's consensus. Validators earn rewards for honest participation and can be slashed (lose part of their stake) for misbehavior.
On Ethereum, becoming a validator requires depositing 32 ETH to a smart contract and running validator software (Prysm, Lighthouse, Teku, Nimbus) on a node that stays online. Each epoch (~6.4 minutes), validators are randomly selected to propose blocks; all active validators attest to the head of the chain. Misbehavior (signing two conflicting blocks, going offline for too long) reduces or destroys the stake.
Validator economics: stake is illiquid for an unbonding period (Ethereum: 1-2 days). Rewards are roughly 3-5% APR depending on total stake. Hardware costs are modest (a few hundred dollars per year of compute).
Validators are the security backbone of every PoS chain. Their distribution (geographic, software-client, operator) determines how decentralized the chain actually is. Concentration of stake among a few large operators is one of the live debates in PoS design.
The exchange-flows endpoint tracks flows to and from major staking pools and validator contracts.