A trimmer cap is a piece of plastic that fits onto a grass trimmer to protect the trimmer's spool line. These caps usually come provided with the grass trimmer, but can also be purchased separately as replacement items.
Grass trimmers are used to cut areas of grass that are difficult for a lawnmower to groom evenly, such as the perimeter of a lawn where the grass grows unevenly or tight spaces. Trimmers come in both electric- and gasoline-powered options. Gasoline-powered trimmers tend to be more powerful and efficient, while electric trimmers are in general less expensive, quieter and more flexible.