To underline something is to draw a line underneath it, usually a word. The reason for underlining words is to show their importance. You can also use the word "underline" to indicate that a situation is important or needs attention.
The word "attention" is underlined.
The teacher told the students to underline all of the verbs in the sentence.
The verbs are underlined.
Ronald underlined an entire paragraph in a book he’s reading because he was so impressed by the ideas he found there.
The titles of the books are underlined on the reference page.
Some hyperlinks are underlined to show that they lead to another page or another website on the internet.
The link is underlined when you roll over it with your mouse.
Growing traffic problems in our area underline the need for new mass transit solutions.