Here's a breakdown:
* Big fish: Represents someone with significant influence, skills, or resources.
* Little sea: Symbolizes a small, restricted, or less competitive environment.
Example:
* "He's a big fish in a little sea in this small town. Everyone knows him and respects his opinions."
This idiom suggests that while the person might be considered important in their current context, their influence or power might be limited if they were to move to a larger, more competitive environment.