Acronym for Proof of Work. A consensus algorithm developed to secure a blockchain by having computers in the network (miners) work to solve a cryptographic puzzle repetitively consisting of the mathematical function (hash).
Miners compete against each other to complete transactions on the network and get rewarded.
Take Bitcoin as an example, the first computer or network of computers (i.e., a mining pool) that found a hash with specific properties are rewarded with a fixed number of Bitcoins roughly every 10 minutes. This is a proof that they have done the necessary work to verify all transactions in a block as valid and are allowed to add it to the blockchain.