Validators are participants on the network that run nodes. Duly called Validator Nodes, these both propose and attest blocks within a Proof of Stake blockchain. This is done by staking crypto on the network and makes them available for random selection in proposing a block.
Afterward, other validators attest that they have indeed seen the block. Once enough attestations have been collected, the block is finally added to the blockchain. The Validators then receive rewards for proposing blocks successfully and producing the needed attestations regarding the detected blocks.