A grease fitting is a metal fitting used in mechanical systems to feed lubricating grease into a bearing using a grease gun. The fitting is permanently installed by a threaded connection, leaving a nipple connection that the grease gun attaches to. The pressure supplied by the grease gun forces a small captive bearing ball in the nipple to move back against the force of its retaining spring. The arrangement is essentially a valve that opens under pressure to allow lubricant to pass through a channel and be forced into the voids of the bearing. When the pressure ceases, the ball returns to its closed position.
Keep your parts and tools greased and in good working order! It'll save you time, money, and potential headaches down the road.