Various carts--i.e. shopping carts and or baskets--strapped together for use of homeless people to transport their worldly possessions. The longer the cart, or bum chariot, the more materialistic, and thus, un-bum-like the individual is to be considered. A true bum chariot is a group of one or more carts attached to a bicycle, where the homeless person in question would be considered the driver, and the useless crap he has for whatever reason assembled would be considered the chariot itself.
Joe: Oh, no. Here comes Smelly Pete.
Jill: Wait...is he on a bike?
Joe: Oh, my God! He is! He's got himself a bum chariot!
Jill: Way to go, Pete!