Property is theft?

OK, time to expose my ignorance again. Who can tell me WTF "property is theft" means & why?

I've heard it for years, but having just come across the phrase again, it strikes me that while I know what it means, I can't quite get my head around why they say it means that.

The mere fact of owning something is theft because it denies someone else? But theft from whom? Wouldn't it have to be someone else's property in order to be theft & that would make them thieves too?

Head hurts.

Here's where I saw it:

And here's what Wikipedia says (Marx - Karl, not Groucho - agrees with me.