It controls the fuel tank vent valve, which clamps around the filler pipe underneath the fender. Pressing down on the tab forces the valve open allowing the tank to vent through the charcoal canister. I replaced my vent valve because I thought it was the reason why my car wouldn't go over 3/4 full. This is what the vent valve looks like.
The valve clamps directly underneath the fill cap.
The little black gizmo with the wires attached is a reed switch believe beileve it works magnetically to detect when the flapper valve in the fill pipe is opened. When the switch closes the a separate vent valve in the tank opens up to release the air and permit the gas intolow inot the tank from the fill nozzle. If the reed switch fails you won't be able to put any gas in the car. But if you depress the little tab shown in your picture you can force open the vent valve and gas up. I found this out when I failed to put the reed switch back in the right position after replacing my vent valve.
The other components of the vent systems are a pair of valves in a housing under the right fender and a charcoal cannister also behind the fender liner. Tsolenoidavalvenoid vavle near the throttle body and the DME uses all those valves to recycle fuel tank vapors and separately to detect leaks in the vent system.