A Validator (or Validator Node) is part of a larger group of validators or nodes operated by distinct entities within a blockchain network. Their main responsibility is to follow and execute the consensus mechanism on the blockchain’s peer-2-peer network. This is the process of ‘validating’ transactions and storing them on ledgers or blocks.
Depending on the consensus mechanism, the node operator has either been selected for its healthy & active participation on the network (like on the XRPL) or its computational and resource commitment toward the validation process (full node on Bitcoin Network & Ethereum). Each blockchain has its own nuances which are conditional to the consensus mechanism used.