the.com/highlight

means The brightest or most noteworthy part of something, or the act of marking something to make it stand out.

from Rooted simply in 'high' plus 'light' — painters and drawers used the term literally, for the bright spots where light hits an object hardest, as opposed to the shadows. By the 19th century it had leapt off the canvas into language, naming any standout moment or detail. The marker-pen sense — swiping a neon stripe across a sentence — is a 20th-century extension of the same idea: making the important part glow.