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

private readonly ICollection _items;

Does not prevent items from being added. This simply prevents _items from being re-assigned. The same is true for _metadata. Accessible members of _metadata can be changed - _metadata cannot be re-assigned.