What is about KISS principle?

I am familiar with the KISS principle in software programming, although I don't really understand what it is about in practice. I could find few articles regarding this which rely on quite different examples and statements. So would like to clarify in details.

1. Case 1.

   As I understand removing of redundant abstractions can be an example of KISS. But from the other side we are also following open/closed and want our application to be extensible so here some abstractions may be needed. Can anyone clarify this?

2. Case 2.

   I am writing some method - the code of the method can be quite complex from point of reading - with some long expressions etc. Is this example of KISS violation ? Or it is about maintainability of the method ?

   Please provide your examples where you are sure KISS is violated or not.