how a function can produce the desired output if one of the parameter requirement is an interface?

For example, in Retrofit package in kotlin or java it is used in network call i.e making a HTTP request to a server on the internet.

So, one of the functions in this package required me to pass an interface which have functions in it that will return a specific object type that Retrofit has already defined in it's package. This object is called Response<T> with a Generic type as it's attribute/field.

The problem is why they wanted an interface but not a class. Because interface as i understand it, is that it's function cannot have a body i.e a definition. It can only have a declaration.

Does that means somewhere in their code they have a function that takes an interface which is defined by me and then they inherits from that interface that i supplied after that return or do what they wanted to do in that function i.e a void function.

so my question is, is this correct? or what actually they are doing with that interface ? because they need to inherit first and then instantiate. if this is correct then what sort of design pattern is this being called?

1. what is the design pattern used?
2. what they do with an interface that not yet have a definition?