Proof Of Work. Before any transaction is recorded in the blockchain, consensus needs to be reached by the involved parties ... this can be done in a few ways, one of them is called POW.
Currencies that rely on the POW consensus need miners who dedicate their computing power to solve complicated mathematical operations. Every block of the blockchain requires certain amount of shares to be found/solved, before it get's completed. Once it's completed, miner who found the last share get's a reward ... this keeps miners interested.
So, the mining devices have to do something in order to complete the blocks ... they have to WORK for it ... that's why this consensus mechanism is called Proof of WORK.