Staking means locking cryptocurrencies to secure the underlying blockchain network and receive rewards. It is an essential part of the Proof of Stake (PoS) consensus mechanism, which is a much more energy efficient alternative to Proof of Work mining.
Consensus happens when all computers on the network agree on the account balances and transactions happening between them. For processing network messages and verifying them until consensus is reached, the computers - called Validators - are rewarded by the protocol.
