The mempool, or memory pool, is where Bitcoin transactions wait before being confirmed and recorded immutably on the blockchain. Each Bitcoin node has its own mempool that stores these pending transactions after verifying their validity. This verification includes checking for double spending and proper cryptographic signatures.

Transactions remain in the mempool until a miner selects them for inclusion in a new block. The size and content of the mempool vary, affected by network activity, transaction fees, and node policies. For instance, when you send a Bitcoin transaction, it enters the network's mempools and remains there until mined into a block, at which point it's confirmed and removed from the mempools.

Users can view Bitcoin's mempool in real-time using the open-source mempool.space tool, in addition to searching and viewing transactions.