A mempool (a contraction of memory and pool) is a cryptocurrency node’s mechanism for storing information on unconfirmed transactions. It is a “waiting area” for Bitcoin transactions that each full node maintains for itself. After a transaction is verified by a node, it waits inside the Mempool until it’s picked up by a Bitcoin miner and inserted into a block.
When a transaction is broadcast, it is sent from a node to its peers, who will then pass it on to their peers. This continues until the transaction has been widely propagated, ready for miners to add it to a block. It’s vital that this buffer zone exists, as transactions are not added to the blockchain immediately.