The Spool Cover holds the trimmer line in place while you work. Its purpose is to hold the spool with the cutting line, in the spool housing and it also acts as the bumper to advance the line from the spool. Easy to use - simply remove this spool cap from the trimmer head by pressing on the side tabs. From time to time you may need to replace your trimmers spool cover, due to general wear and tear or if you accidentally catch the spool cover against a wall or stone. Please check model fit list to ensure this part is correct for your model.