A blockchain validator is a person responsible for confirming and verifying transactions on the blockchain. A blockchain is a decentralized public ledger for the record of transactions. As a result of the decentralized nature of cryptocurrencies, there is no centralized authority responsible for verifying transactions.
Validators operate based on the underlying blockchain consensus model. For a blockchain that works on proof-of-work, the validators are miners and use high-end computational power to solve complex puzzles. For the proof-of-stake consensus model, validators stake assets and verify transactions based on the asset staked.
Validators are given rewards in the native token of the network. Thus, validators are popularly called node operators. As transactions occur, new blocks are added to the chain. The roles of validators are essential because, without validators, there will be no blockchain.