A “Mining Pool” is an organized syndication of individual & independent mining entities that pool their computational power and share the extracted mining rewards accordingly.
With an every increasing competitive mining landscape and non-deterministic structure of mining itself, individual mining become more complex and less profitable. By joining a pool rewards a certainly fractional but they are much more consistent.
Pools appear in 2 forms, public and private; public pools are open and can be joined at any moment & private pools are invitation only closed environments.