The phrase "big fish" is an idiomatic expression that means someone who is important, influential or powerful. It can also refer to a person who has achieved great success in their field or career and is considered a leader among others. The term comes from the idea of catching a large fish with a hook, which requires skill and patience, and implies that the person being referred to as "big fish" is someone worth pursuing or paying attention to due to their significance or importance in a particular context.