When you start the engine, a small gear, known as a pinion, is pushed along the starter's shaft to the flywheel. Teeth on pinion engage teeth on a gear around the flywheel. The starter motor turns the pinion, the pinion turns the flywheel, which turns the engine.
But, if the engine is already running when the starter is engaged, the teeth on the flywheel and pinion won't mesh. They will collide or grind against each other. That sometimes can break teeth on the pinion, flywheel, or both.