Bum is a slang term for your bottom - the part of you that supports your weight when you are sitting. It has many other names like your backside, buttocks, arse, buns, butt , fanny etc. Your bum is covered with skin and under that a layer of fat and under that muscles.
It can play a large part in sexual attraction. But the size of a person’s bum which is considered to be a ‘turn on’ varies from one culture to another and from one sex to another!
Other meanings of the word ‘bum’ include: someone who is homeless, someone who is lazy, someone who tries to obtain something for free.