It is the street term for the psychoactive substance derived banana peels. It is smoked, inhaled, or injected for an intense high between LSD and DMT. Supposedly US Soldiers used to smoke "banrack" in Vietnam.
A banana is treated with a series of lighter fluid and ether for a span of a few days, then allowed to dry. Then you have banrack, and it is smoked. There are often several other methods, and other ways it is smoked similar to crack.