It is what it is.
I’ve heard it used in two almost opposing ways.
As a starting point: Begin with how things are and grow from this reality a new and better one.
As the last word: Don’t bother to put in any effort because what is cannot be changed (and I’m not responsible for trying).
The first doesn’t deny reality; it confirms the way things are and affirms that it can be changed for the better. It’s a great truth.
The second doesn’t deny reality, either: it denies that it can be otherwise. In denying the mutable nature of all things, it’s one of the biggest falsehoods ever spoken.