The pool is a service which provides joint production of cryptocurrency. Pools became the natural phenomenon with the growth of complexity of mining. The sense of a pool is that the group of miners by means of a special software begins to perform work on the search of the new block together.
The pool divides all operation into small parts which are in turn distributed between all participants. Collaboration allows to increase considerably chances of the finding of the block, and the reward is in proportion shared between all participants.