The Mempool is a “waiting area” for unconfirmed Bitcoin transactions which is maintained at each full node. Each node can have a different mempool from the others and it works as a staging area for transactions that have happened but are waiting to be added to the blockchain.
Every new transaction performed by the node is recorded in its mempool and broadcasted across the network. It is first relayed to the closest nodes which check the transaction to make sure it has a valid signature and other parameters. Then they relay it further until it is added to every single mempool in the network.
The mining node picks from the mempool the pending transactions to put in a block. Usually, a mempool in a Bitcoin network contains an average of 7,000 transactions but it can go all the way to 16,000 transactions during the day, and each block can contain an average of 2,000 transactions.