shes not the only fish in the sea

Shes (or hes) not the only fish in the sea is another way of saying that this person is not the only one of their kind and that there are plenty of others just like him or her to choose from. This phrase is often said to someone who is upset about a recent break-up and you want to reassure them that even though it didnt work out with that particular person, it doesnt mean it wont work out with someone else.