It means that the list can only contain items of the String type. It can't contain items of Number, SomeObject, WhateverType types.
It's called Generics. In Java, it's actuallty compile time syntactic sugar to make the code more robust without the risk for ClassCastException and consorts on retrieving the list items during runtime. You can find here a Sun tutorial on the subject, it also explains the reasoning behind the move to Generics.
