The mempool, short for memory pool, is the waiting room for Bitcoin transactions. When you send bitcoin, your transaction does not go straight into a block. It first sits in the mempools of nodes across the network until a miner includes it.
There is no single mempool
Each node keeps its own mempool in memory. They are usually very similar because nodes relay transactions to each other, but they are not identical. Nodes can have different size limits and policies. By default, Bitcoin Core limits its mempool to 300 MB and drops the lowest-fee transactions if it fills up.
A fee market
A block has limited space, roughly 4 million weight units. When more transactions are waiting than fit in the next block, miners normally choose the ones paying the highest fee per virtual byte (sat/vB). During busy periods, fees rise. During quiet periods, even 1–2 sat/vB can confirm quickly. Fee estimators and public mempool explorers help you choose a fee.
Stuck transactions
If your fee was too low, your transaction can wait for hours or days. Two tools can help:
Replace-by-fee (RBF): broadcast a new version of the transaction with a higher fee.
Child-pays-for-parent (CPFP): spend the unconfirmed output in a new transaction with a high fee, so miners take both together.
By default, Bitcoin Core drops unconfirmed transactions after two weeks (336 hours). Your wallet may still need to abandon or rebroadcast the transaction before the coins show as spendable again.
From mempool to block
Once a miner includes your transaction in a block, it gets its first confirmation. Until then, it is a promise, not a settlement, and it can still be replaced. That is why recipients of large payments wait for confirmations before treating the money as theirs.