Mempool

The mempool (memory pool) is the waiting area for transactions that have been broadcast but not yet included in a block. Each node keeps its own mempool, so there is no single official one, though popular block explorers show a typical view.

When demand for block space is high, the mempool fills up and miners pick the transactions paying the highest fee per unit of data first. That is why fees spike during busy periods and why a low-fee transaction can wait hours or even be dropped.

If your bitcoin transaction is stuck, you may be able to speed it up by replacing it with a higher-fee version (replace-by-fee) or by spending its output with a high-fee child transaction (child-pays-for-parent), depending on your wallet. Checking mempool congestion before sending helps you choose a sensible fee.