What is a "Christian-Atheist"?

Someone that I know was talking about a book they are reading by the same title. I googled it, but I still don’t understand. The information that I was finding seemed conflicting. The book says “Believing in God But Living As If He Doesn’t Exist”, but somewhere else I read that it means following Jesus’ teachings while rejecting the existence of god.

What??

I feel incredibly stupid, I can’t wrap my head around this at all.