[DotNetDevelopment] Interface, what can you really do ?

[It's time to do something]

Hi, can anyone here give me some ideas on how important interface is
(in C#) and what it can do?
I've learned that it can  be used as polymorphism, but I don't
understand why in some cases we don't use polymorphism instead ? And
when do we use interface ? When do we use polymorphism  instead ?
Thank you so much ! I'm just a newbie at all. Hope someone will help
me !