How to comprehend that the type of `std::function` depends solely on its call signature?

As per the documentation(http://www.cplusplus.com/reference/functional/function/), which says that:

Class that can wrap any kind of callable element (such as functions and function objects) into a copyable object, and whose type depends solely on its call signature (and not on the callable element type itself).

How to comprehend that the type of std::function depends solely on its call signature(and not on the callable element type itself)? Could somebody make it clear by giving some simple examples?I would be greatful to have some help with this question.