Validator nodes are entities in a blockchain network that are responsible for verifying the legitimacy of transactions and adding them to the blockchain. They operate within proof-of-stake (PoS) or delegated proof-of-stake (DPoS) consensus mechanisms, whereby they are selected based on the amount of cryptocurrency they hold and are willing to 'stake' as collateral. The validators play a key role in preventing double-spending, ensuring that the transactions are properly ordered, and adding new blocks to the blockchain. They can earn rewards for their services, making it a vital process in blockchain economics.