Berkeley thought that the world and everything in it only exists when somebody is observing it. When we stop looking at something it no longer exists. This theory is called Idealism. Not to be confused with “being an idealist,” which usually means something else. And it’s neatly captured by the phrase “esse est percipi;” “to be is to be perceived.” That’s how my old textbooks used to explain it, I think that’s a little bit blunt: Berkeley’s point is actually a little subtler than that. I think the best way to get at what he’s saying is with a challenge.