What is a Bitcoin chain reorganisation (reorg)?

Competing valid blocks can appear at the same height. Nodes follow the valid chain with the most accumulated proof of workProof of workA puzzle that takes a computer real effort to solve, so the answer proves the effort was spent. Bitcoin’s miners use it to add each block., so a previous chain tip can be replaced.

A transaction in a replaced block may return to the mempoolMempoolThe waiting room for Bitcoin transactions that have been sent but aren't in a block yet. or appear in the new chain. More confirmations make this kind of reversal harder; zero confirmations is not final either.