A lip liner is a makeup pencil designed to outline and define your lips before applying lipstick or lip gloss. Its main functions are to:

• Define the lip shape for a sharp, polished appearance.

• Prevent lipstick from bleeding or feathering outside the lip line. Wikipedia

• Create the illusion of fuller lips when applied slightly outside your natural line. Wikipedia

• Extend the wear of lipstick, acting as a base that helps color last longer.