Mempool
In short The mempool is the waiting area where valid but unconfirmed transactions sit until a miner or validator includes them in a block.
The mempool, short for memory pool, is the set of pending transactions a node has received and checked but that are not yet in a block. When you sign a transaction, it is broadcast to nodes, which keep it in their mempool and pass it along. Each node has its own mempool, so the exact contents can differ slightly across the network.
Block producers usually pick transactions from the mempool that pay the most, so fees work like an auction for limited block space. On Ethereum and EVM chains you can raise the priority fee to move up, or send a new transaction with the same nonce and a higher fee to replace a stuck one. Bitcoin supports fee bumping too. Solana works differently: it has no public mempool in the same sense and forwards transactions straight to upcoming leaders, though priority fees still apply.
For traders, the public mempool is where front-running happens. Bots watch pending swaps and can place their own trades just before and after yours, a sandwich attack that pushes your price to the worst level your slippage allows. This is a form of MEV. During busy launches the mempool fills up, fees spike, and low-fee transactions can sit pending for a long time or be dropped.
Keep slippage settings as tight as practical on volatile tokens, and consider wallets or RPCs that offer private transaction submission to keep swaps out of the public mempool. If a transaction is stuck, speed it up or cancel it with the same nonce instead of sending duplicates. Remember that a pending transaction is not final until it lands in a block.