The mempool is a short hand description for the Bitcoin memory pool. The memory pool is where transactions wait before miners place them in blocks and mine them. In other words, the mempool is where transactions wait before they get confirmed.
This process takes an average of 10 minutes, but it depends entirely on the demand within the mempool. The demand, or size, can fluctuate wildly depending on the number of transactions that are entering the network.
The mempool is like a highway for Bitcoin
When highways have traffic, travel takes more time. The Bitcoin Mempool works similarly. At high demand, it is difficult for miners to select which transactions will get into a block, which means higher wait times to have transactions confirmed. At low demand, miners select virtually all transactions without the user experiencing wait times.