What is the parameter "in" in a generic type in C#?

Question

I came across a C# scenario that I didn’t quite understand. I was studying and I saw a code piece with a generic type that has the reserved word in for that guy.

What would be the purpose of using this in in the Generic Type declaration?

Code example:

public interface IHandler<in T> where T : Message
{
    void Handle(T message);
}