Definition
This idiom is used to comfort someone who has ended a relationship, suggesting that there are many other potential partners.
I know you're upset about losing this job, but don't worry, there are plenty more fish in the sea.
She was feeling down after breaking up with her boyfriend, and her friend comforted her by saying, "Don't be sad, there are plenty more fish in the sea."
You just got rejected by the university, don't be too disheartened, there are plenty more fish in the sea.