The process of mining cryptocurrency blocks is extremely resource intensive in many cases, and can use up a lot of computer processing power. It is also very difficult to mine blocks on some blockchains, and individual miners often do not have enough computing power to statistically have any chance to mine a block on their own.
To get around this, mining pools were formed, which pool the resources of a group of individual miners. This makes it possible for the pool to have greatly increased mining power, and the ability to solve and mine blocks. When the pool does mine a block the blockchain reward is split evenly between the participating members of the pool.