So I've been assigned to talk about adding new code routines into a program such as self contained functions and new classes, yet I haven't actually been taught this kind of programming terminology yet. I've tried looking online everywhere but it doesn't really explain it well enough.

The questions that I'm a little confused by are the following:

What are self contained functions in C#? (Code examples would help :3)
And how could it be added in a object-oriented way?

Help would be very appreciated, thanks.