Mempool

For proof of work blockchains like Bitcoin. The mempool is the waiting room were nodes hold unconfirmed transactions while they are pending addition to a new block by miners. Transactions are verified when they are pulled from the mempool by the miner who successfully proposes the next block confirmed by the network.

When a network receives more transaction requests than it can handle. Fees rise as it becomes more expensive to get miners to add transactions to blocks. With lower value transactions sometimes pending for weeks.

This can happen because nodes may agree to impose a minimum fee in an attempt to clear network congestion. Transactions inputted but not confirmed before the imposition of the fee will often languish if they don’t meet this minimum threshold. Because miners won’t pull them into blocks until the network is less congested and the minimal fee threshold is removed.

Mempool Resource

Johoe’s Bitcoin Mempool Statistics