Immutable vs Unmodifiable collection [duplicate]

From the Collections Framework Overview:

Collections that do not support modification operations (such as add, remove and clear) are referred to as unmodifiable. Collections that are not unmodifiable are modifiable.

Collections that additionally guarantee that no change in the Collection object will be visible are referred to as immutable. Collections that are not immutable are mutable.

I cannot understand the distinction.
What is the difference between unmodifiable and immutable here?