Mempool
The waiting area of transactions that have been broadcast to the network but not yet included in a block.
Every node keeps its own pool, applies its own policies about what to accept and how long to keep it, and evicts entries when the pool fills, which is why two block explorers can honestly report different pending sets. Block producers select from what they hold, generally taking the highest fee per unit of gas, so the pool is where fee competition actually happens. Because the public pool is visible to anyone, a pending swap can be seen and traded around before it executes, which is the raw material for sandwich attacks and other extraction; this is why some users submit through private channels straight to builders instead of broadcasting. Congestion appears as a growing pending count and a rising base fee.
In practice
A transaction that offers less than the current base fee is not eligible for inclusion and may sit in nodes' pools until it is dropped or replaced.
The common misunderstanding
There is no single global mempool; each node keeps its own view, so pending-transaction figures differ between sources and are not authoritative.