A mempool, short for “memory pool,” is a temporary storage area in a blockchain network where unconfirmed transactions are held before they are added to a block. When a user sends a transaction on networks like Bitcoin or Ethereum, the transaction is first broadcast to the network and placed into the mempool of various nodes.

Each node maintains its own version of the mempool, which contains pending transactions waiting to be validated and confirmed by miners or validators. These transactions remain in the mempool until they are selected and included in a newly mined block. Once confirmed, they are removed from the mempool and permanently recorded on the blockchain.

The mempool plays a crucial role in determining transaction speed and fees. When the network is busy, the mempool can become crowded, causing delays. In such cases, users often attach higher transaction fees to incentivise miners to prioritise their transactions. As a result, miners typically choose transactions offering higher fees to maximise their rewards.

Additionally, if a transaction remains in the mempool for too long due to low fees, it may be dropped from the pool altogether. Overall, the mempool acts as a waiting room for transactions, helping organise and manage the flow of data before it is securely added to the blockchain ledger.