means A marker filled with bright, translucent ink used to draw attention to important words on a page without obscuring them.

from From 'highlight,' which began life in the visual arts: the 'high lights' were the brightest spots in a painting, where the most light fell. By the 19th century 'highlight' had stretched to mean any standout feature worth emphasizing. The pen itself is a 20th-century invention — fluorescent, see-through ink that lets you 'light up' text while still reading it — and 'highlighter' simply names the tool that does the highlighting.