implementations implement the interface members. interface doesn't really do much other than have some items, which you can't change in the code. while implementation can change data of the items in it.
You are on the right track, but try to think of interface in this question as WHAT something does, while implementations are HOW something does it. What are the repercussions of changing WHAT something does as opposed to HOW it does it?
Imagine you write an application that uses code that I have written. What happens to your application if I change HOW my code does something? What happens to your application if I change WHAT my code can do?