When someone is the most skilled at an activity out of a relatively small or otherwise novice group of individuals in regards to said activity. Often used to denote that the "big fish" is not as skilled as those inside of the "small pond" would make it appear. Commonly used as an insult to the person being described.
Jim: I am the smartest one in my class!
Hank: Yeah, but you go to community college so... big fish small pond.
Sally: Laqueesha is the best prostitute I know!
Susan: That's because she lives in Old Lyme. If she lived in Compton she wouldn't be in the big fish small pond scenario.