That's the charcoal canister. It's supposed to catch fuel vapor from the tank, trap it, and feed it to the engine to burn. If it's duct taped, good chance it's not actually working. Probably not a fire hazard, but I'd still either remove & bypass it (If you live somewhere without emissions inspections) or replace it with a junkyard one (if you do have inspections).
Edit: correction: it sends the fuel vapors into the intake to be burned, not back into the tank.