Nonce

A nonce ("number used once") is the dial miners spin in proof of work. A miner takes the block's data, adds a nonce, and runs it through a hash function. If the result is not small enough to meet the network's target, they change the nonce and try again. And again. Trillions of times a second across the network.

Whoever lands on a winning nonce first gets to add the block and claim the reward. It is pure brute-force guessing, and that is the point: finding the answer is hard, but checking it takes a split second.