A mempool is a node’s local collection of valid, unconfirmed Bitcoin transactions. Because each node applies its own policy and receives transactions at different times, there is no single universal mempool.
From broadcast to confirmation
Nodes relay transactions that pass their validation and policy checks. Miners select from the transactions they know about when assembling candidate blocks.
Every node has its own pending-transaction view
Nodes receive transactions at different times and may use different relay policies, so mempool size and contents can vary across the network.
Fees affect selection, not validity
A higher fee rate can make a transaction more attractive to miners during congestion, but it does not guarantee a precise confirmation time. Unconfirmed transactions can also be dropped from individual nodes over time.
Miners compare fee rate with available block space
Transaction size and fee together determine fee rate. During congestion, low-fee transactions may wait behind transactions offering more per unit of block space.
Sources
Educational information, not investment advice.