Mining is a guessing game with no shortcuts. A miner takes a block, adds a number called the nonce, and runs it through SHA-256 twice. If the result is below the target, which looks like a hash starting with enough zeros, the block is valid. If not, change the nonce and try again.
Press Start and your own device plays that game. Each extra zero you ask for makes the search 16 times longer on average. Then compare your speed with the live network: the gap is the security that protects every coin, because rewriting history means redoing that work faster than everyone else.
The default text is the headline Satoshi Nakamoto put in the first block in 2009. This is a teaching model: real miners hash an 80-byte block header, and the real target is set by the difficulty, not by counting zeros.
Related lessons: Mining and proof of work, Keys, hashes and signatures, The difficulty adjustment.