It literally translates to "fifteen years", but it can be used in different contexts:
* Age: "Il a quinze ans." (He is fifteen years old.)
* Duration: "J'ai vécu ici pendant quinze ans." (I lived here for fifteen years.)
* Anniversary: "C'est notre quinzième anniversaire de mariage." (It's our fifteenth wedding anniversary.)
Let me know if you have any other questions about French!