Bitcoin has millions of daily users. Therefore, there needs to be a way for all these transactions to be organized. That’s where the Bitcoin mempool comes in.
Transactions made on the Bitcoin network aren’t added to the blockchain immediately. Instead, they are collected and stored in a temporary waiting area called the mempool.
Every full node in the Bitcoin network has its mempool, where it stores a list of transactions it has checked and considers authentic. In a nutshell, all Bitcoin transactions first exist as ‘unconfirmed’ transactions in the mempool before they are permanently recorded on the Bitcoin blockchain.
The Bitcoin Mempool feature was added to the Bitcoin network after the BIP 35 (Bitcoin Improvement Proposal 35) by Jeff Garzik was approved.