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:

What if I have a collection and mark it as readonly, like the following. Can I still call _items.Add?
```
private readonly ICollection _items;
```
And also for the following variable, if later on I call _metadata.Change which will change the internal values of a couple member variable in the Metadata instance. Is _metadata still immutable?
```
private readonly Metadata _metadata;
```

For both variables above, I totally understand that I can't directly assign new values to them outside of initializer and constructors.