A “Mempool” is a temporary storage location of a transaction that is pending confirmation. Whenever a transaction is submitted to a blockchain it is received by the consensus node and checked for its validity. Once the transaction is verified by the receiving node as being valid is it put into a “mempool” and awaits to be included in the next possible block.
Each node typically has its own Mempool that is located locally on their devices, this can cause interesting issues if they become full and cannot accepts new incoming transactions. In these events the nodes begin to prioritize transactions that satisfy their “fee thresholds”.
In newer blockchain environments Mempools are shared between multiple nodes (to help guarantee fair transaction execution).