Primaries question

I live in North Dakota where I vote Dem in the Primaries. I was looking at this website and it said the Dems are open but the Republicans are closed.

Maybe my googlefu is weak, or maybe I'm phrasing stuff wrong, but I generally hate to start "asking questions" threads. I can generally find most of the information pretty well, or at least eventually, online. Can anyone explain why the Dem primary is open but the Republicans is closed?