A mining pool is a way of being able to mine cryptocurrency quicker by multiple miners working together rather than individually.

Normally you would mine cyrptocurrency from your own machine and it might take a while before you receive any currency as it might take a while before the calculations required to mine a block are completed. What the mining pool does is distribute the processing power needed to mine each block. This means that blocks can be mined quicker and the appropriate amount of currency is allocated depending on how much work your machine has put in.