There is no single mempool — each node keeps its own — but they converge on the same picture: a queue of unconfirmed transactions bidding for block space by fee rate (sats per vbyte). Miners take the highest bidders first.
Reading the mempool is a practical skill: send during a lull and pay a tenth of peak fees. Sites like mempool.space visualize the queue in real time — the fee ticker in our site header comes from exactly that data.