Proof of work

Proof of work is how Bitcoin decides which blocks count. To propose a block, a miner must find a header whose hash falls below a target set by the current difficulty. Finding one takes an enormous number of attempts. Checking it takes a single hash.

The asymmetry is the point. Rewriting history means redoing all the work that came after, which grows more expensive with every block. Nodes follow the valid chain with the most accumulated work. Holding bitcoins gives no vote in this process, and miners propose blocks without deciding the rules those blocks must obey.