Proof of Work (PoW) is a consensus algorithm used in the original Bitcoin implementation. In a Blockchain system new transactions are periodically added by packaging these transactions in a block. This block is then added to the Blockchain.
Please read What is Blockchain Technology if you don't already know.
Users send each other digital tokens (e.g. Bitcoins). All such transactions are stored in a public ledger which implemented on Blockchain. But before transactions are added to the ledger they need to be confirmed. There are special nodes in the network called peers or more popularly miners who are responsible for confirming transactions and adding them to blocks. A block gener