Literally, your ass is your buttocks. In slang, an ass is a jerk or an idiot. Neither is highly regarded.
When people are being slangy or folksy, you hear this word a lot: "Get off your ass!" "Watch your ass!" "Why are you such an ass?" "Get your ass in here." Though this isn't one of the very worst swear words, it is considered harsher than butt and lots of people consider ass to be an inappropriate word. You would be a lot better off using softer words like bottom or tush in most situations. Sometimes ass can also mean sex — another use worth avoiding.