The screw is the idle stop screw. It adjusts how far your carb can close down. If you screw it way in, the carb will not close.

If you are flying a trainer and are new to RC, you are probably running too lean. Our engines are simple things without fuel pumps. Fuel is sucked into the carb by venturi action and the fuel level in the tank can change the needle valve setting. Your engine needs to be set slightly rich at the first of a tank or it will go lean towards the end of the tank, espacially if the tank is low relative to the carb. Trainers are nortorious for having low tanks, causing the engine to run lean at the end of the flight.

Fill your tank about 1/3 full, no more. Crank up and lean out to max power, then open up a couple or 3 clicks. Stop the engine and fill the tank. You'll probably notice the engine is running slightly rich. This is the setting you need to start with to have the engine run well for the full flight.