Proof-of-work (PoW) is a consensus algorithm used to validate the transactions that occur on a blockchain network. It is used by many decentralized networks to prevent malicious activities such as double spending and Sybil attacks. In PoW, miners use their computing power to solve math problems, which verify the validity of the transaction before it is added to the blockchain ledger. When a miner successfully solves the problem, they are rewarded with cryptocurrency.