Proof of work is the consensus mechanism by which bitcoin transaction are verified on the blockchain. When a transaction takes place, it is broadcast on the network, packaged together with other in a block. Each block comes with a set of cryptographic rules (complicated mathematical functions) called a hash that miners must work to verify. These rules also make it very hard for anyone to manipulate or fraudulently add or remove blocks from the blockchain. The first miner to successfully verify the block is rewarded. But in order to claim their reward, the miner must prove they carried out all the computational effort required or in short show proof of their work.