When you send crypto, your transaction does not jump straight into a block. It first enters the mempool, a temporary waiting room on each node where unconfirmed transactions are stored and checked.

This mempool layer is why wallets show “pending” even though you already signed and broadcast the transaction.

Every full node keeps its own mempool, constantly receiving new transactions from the network, validating them, and discarding invalid ones. 

Only transactions that pass checks such as a valid signature, correct nonce, and enough balance stay in this pool and become eligible to be picked up by miners or validators.

During busy periods, the mempool fills up and turns into a live fee market.

Nodes rank transactions mainly by fee per unit of gas or byte, so higher‑fee transactions are more attractive and are usually selected for the next block first, while low‑fee ones may wait longer or even be dropped if they stay uncompetitive.

Understanding the mempool helps explain why two users making similar transfers can see very different confirmation times. 

The one paying a higher fee effectively jumps the queue, while the other remains in the mempool until network demand falls or the fee is manually raised with a replacement transaction, if the chain supports it.

For everyday users, watching mempool size and typical fee levels offers a clearer picture of network congestion than price charts alone. 

Tools and explorers that visualize mempool activity let you choose smarter fee settings, avoid sending non‑urgent transactions at peak times, and understand exactly what is happening when a transaction seems “stuck” but not yet failed.