The British word "arse" is the same as the American word "ass". Both words have two meanings.
1) You "arse" or "ass" is your bum, butt, buttocks - the part of your body that you sit on. For example:
I fell on my arse.
Get up off your arse and do something!
2) If you call someone an "arse" or an "ass" it's a mild insult. You're saying he's a bad person and you don't like him. For example:
You're such an arse!
Don't be such an arse!