If something is detached, it stands apart from something else. That goes for people, emotions, retinas, garages, and just about anything else that can be disconnected or uninvolved.
In the simplest sense, detached means "not attached." Which is a fine state to be in if you're talking about houses (it means they're not connected, like row houses) or your neighbor's aggressive dog (who was chomping down on your leg a few minutes ago). But if a person is emotionally detached, be careful: They're not in touch with their own emotions, or very interested in understanding yours.