Hi Martin, LaVaughn is correct saying "Stay there" means to remain where you are. "Stay in there" adds the extra meaning of being inside a place while you remain there. I'll just give some examples. Stay in there could be as specific as if someone was told to stay in a room or a broad as staying in a particular country.

thanks a lot .it is very nice of you .That is what i want.

"Stay there" means to remain where you are. "Stay in there" adds the extra meaning of being inside a place while you remain there. Hope that helps! :-)