Proof of Work is a consensus algorithm to achieve a global consensus. The algorithm is used by Bitcoin and some other currencies. In contrast to the Proof of Stake approach, Miners use a lot of energy to calculate hashes.
In a Proof of Work consensus algorithm, computers must solve a specific type of mathematical problem (or millions of them) that takes at least a certain amount of time.
This process is called “mining“. When a miner sends a block to the blockchain, the solution to the mathematical problem is presented along with the transactions embedded in the proposed block. Invalid solutions (which are difficult to calculate but easy to check) are rejected by the other miners in the network. This mechanism encourages rational miners to submit only valid blocks, otherwise they have wasted time and effort.