Definition
The mempool (memory pool) is each node’s local collection of unconfirmed transactions waiting to be included in a block. When a user broadcasts a transaction, it propagates across the network and enters the mempools of nodes and miners.
Miners select transactions from their mempool to include in the block they are trying to mine, typically prioritizing transactions with higher fees. When the mempool is congested (many pending transactions), fees rise as users compete for limited block space. Mempool monitoring helps miners optimize fee revenue and helps users estimate appropriate transaction fees.
In Simple Terms
The waiting area for unconfirmed transactions. Miners pick transactions from here to include in blocks.
The waiting area where unconfirmed Bitcoin transactions sit before being selected by miners for inclusion in the next block.