What is the mempool?

Each Bitcoin node keeps its own list of valid transactions that haven't confirmed: its memory pool, or mempool. Miners can evaluate related transactions together, rather than simply taking the highest individual fee rates first.

Bitcoin Core's default mempool expiry time is two weeks. Expiry removes a payment from that node's waiting list; it does not cancel the payment. Other nodes may keep it, and a wallet may broadcast it again. Until a conflicting transaction confirms, it can still be included later. A payment with zero confirmations is not final.