When one thing is behind another, it's at the rear or the far side of it, possibly even hidden by it. Your shy dog might tend to stand behind you when you meet a friend on your walk.
When the sun hides behind a cloud, the cloud is in front of it, and when there's fear lurking behind your cheerful smile, the grin is disguising, or covering, your anxiety. Behind can also mean "supporting," as when you're a hundred percent behind your sister's campaign for school president; or "lagging," as when you say: "Slow down! I'm way behind in this race!" Finally, behind is a gentle slang term for "buttocks."