mempool

The mempool — short for memory pool — is the waiting room for transactions. When you broadcast a payment, it does not land in a block instantly; first it sits in the mempool, a holding area of valid-but-not-yet-confirmed transactions that each node keeps in its own memory. It is like the queue at a post office counter: your envelope is genuine and ready to go, but it has to wait its turn before anyone actually processes it.

Every full node maintains its own copy of this waiting area and gossips new transactions to its neighbors, so a freshly broadcast payment spreads across the network within seconds. Nodes first check that each incoming transaction is well-formed, properly signed, and not trying to spend money that is already gone; only then does it earn a place in the pool. The mempool is therefore both a buffer that smooths out bursts of activity and a filter that screens out invalid transactions before they could ever reach a block.

When it is time to build the next block, the participant doing so reaches into the mempool and selects which waiting transactions to include — and because block space is limited, they typically prioritize those offering higher fees. Once a transaction is confirmed in a block, it leaves the mempool. This is why a payment can feel pending for a while during busy periods: it is simply still in the queue, waiting to be picked up into a block.