Mining

Mining is how proof-of-work cryptocurrencies like Bitcoin add new blocks of transactions. Miners run powerful machines that guess numbers trillions of times a second, racing to find one that solves a puzzle set by the network. The winner gets to add the next block and collects the block reward plus fees.

The guessing is deliberately expensive. It burns real electricity, which makes rewriting history ruinously costly for an attacker. That cost is the security. See proof of work for the mechanics and nonce for the number they are hunting.