A mempool (a contraction of memory and pool) is a cryptocurrency node’s mechanism for storing information on unconfirmed transactions. It’s like a waiting room of the Bitcoin network.
When a transaction is transmitted to the network, it first gets verified by all of the computers participating in the Bitcoin network. It usually takes about 10 minutes but depending on the number of pending transactions, this interval can be extended to several hours or days.
Each new transaction that has received the approval of the network goes to the mempool, where it is awaiting confirmation from the miners. When a miner takes a transaction to include in the next block, it’s automatically removed from the mempool.
If new transactions arrive at a higher rate than they are cleared from the mempool into blocks, a “traffic jam” will occur and transactions can take a long time to get approved.