A mempool, short for memory pool, holds transactions that a node has accepted for possible inclusion in future blocks. It is a local working set, not one globally synchronized queue shared by every node.
Why views differ
Nodes receive transactions at different times and apply their own admission and resource policies. A transaction visible to one explorer may therefore be missing from another. Some transactions reach block producers through private submission channels and may never appear in the public mempool being observed.
Waiting is not confirmation
Block producers select transactions subject to validity, capacity and their selection rules. On fee-market networks, competitive fee rates can improve inclusion prospects, but queue position is not a promise. Dependencies or an earlier missing account nonce can also delay processing.
Removal and replacement
An included transaction leaves the pending set. Nodes may also remove transactions because of resource limits, expiry policies or accepted replacements. Disappearing from a particular mempool does not prove cancellation: another node may retain or rebroadcast the signed transaction. A chain reorganization can return previously included transactions to pending status if they remain valid. Check the current chain outcome before treating a payment as settled.