Mempool

When a user sends a transaction, it is broadcast to nodes across the network. Each node checks that it is valid and then holds it in memory until a block producer includes it in a block. That holding area is the mempool, short for memory pool. Every node maintains its own mempool, and they differ slightly because transactions reach nodes at different times.

Block space is limited, so when more transactions are waiting than can fit, block producers pick those paying the highest fees. The mempool is therefore a live fee market. During quiet periods on Bitcoin almost everything confirms in the next block; during busy periods the mempool can hold hundreds of thousands of transactions and low-fee ones may wait days or be dropped.

Watching the mempool helps users choose a fee. Sites like mempool.space show the current backlog and estimate what fee rate is needed to confirm within a target number of blocks. On Ethereum, the mempool is also where bots look for profitable trades to front-run, a practice known as MEV.