It is the fuel shut off. Pull it out to stop the engine, push it in, then use the key to start it. Make sure you turn the key off after stopping the engine to keep from running the battery down.
Some tractors came with a decompression knob you can use for cold weather starting and weak batteries. Lets you spin the motor up before actually trying to start it. Generally you dont want to use the decompression knob to stop the engine.
Try finding a manual for your tractor or post a pic on the knob showing the graphic.