It's what transfers the force from the pedals to the engine for starting. Normally it shouldn't just fall off. Drain the transmission fluid, remove the pedal arms, remove the transmission cover. Take the clutch out and then you can slide the pedal shaft and gear cluster out at the same time. Inspect everything for wear or damage. Clean out any broken bits, put everything back together, align the claw collar with the case and reinsert the pedal shaft and gear cluster at the same time. Once its in twist the gear very slightly to make sure it's fully seated. Now put it back together with a new gasket.