Mempool
The mempool is each node’s waiting room of valid transactions that have been broadcast but not yet included in a block.
In depth
When you send a payment it first lands here. Miners pick transactions from the mempool, usually highest fee first, to fill the next block. If you pay too low a fee during congestion, your transaction can sit in the mempool for hours or eventually be dropped.
Why it matters
Checking the mempool tells you why a transfer is slow and what fee would speed it up.