Definition: to be important or famous, but in a small group (note: this is not a positive idiom — if you tell someone they are a big fish in a small pond, they will get angry at you!)
I am a top manager at this company, but the company is really small. I am tired of being a big fish in a small pond — I want to go work at a large international company.
He thinks he is really important, but outside his group, no one knows him. He is just a big fish in a small pond.