Rubberband when used in T.I.'s song "Rubberband Man" refers to the common use of rubberbands (and now the yellow LIVESTRONG-type rubber wrist bands) to signify how much weight of a certain drug they had to sell. X rubberbands meant X amount of cocaine/crack/weed. This is used so that when stopped by police for loitering outside on a corner/lawn at certain time at night you are not caught with narcotics in possession and can warn those in the house/apartment with the illegal drugs to leave or get rid of them.
Fiend #1 - "Yo! the connect has 4 rubberbands on his wrist, you got a $20 spot?"
Feind #2 - "Nah! Here's a $5, get one nic"