What does immutable and readonly mean in C#?

Question

Is it correct that it is not possible to change the value of an immutable object?

I have two scenarios regarding readonly that I want to understand:

Answer 1

The readonly keyword applies to a variable - this means you can't assign another value to it, but you can alter its internal state. That's why you can alter a collection's items but you can't assign a new collection to the variable.