Example of Interface?

I was wondering if someone could provide me an example of how to implement an interface that I could relate to or understand in practice?
The documentation explains that they are methods that must be implemented but overridden, and as I understand it, that’s just defining names of methods but nothing about what they actually do.
Can I define functionality there? Or is there some reason I would want a bunch of functions with expected names, but not functionality?
Thanks.

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