In short, Mempool refers to the space where all Bitcoin pending transactions are being stored. Each node maintains a Mempool. Once the transaction is verified by all the nodes in the blockchain it is stored in the Mempool. Then the miner picks it up and adds it to the block.

But how do the nodes manage an overload of pending transactions in the Mempool? The nodes would set up a minimal transaction fee threshold. Transactions with low transaction fees are removed instantly and that’s how nodes manage to maintain a Mempool. Only the transactions with higher transaction fees remain in the Mempool.