An underline (a line drawn under text) used to emphasize or mark text; also called an underscore/underline in computing contexts.
重要な語句には下線を引いておきなさい。
Underline the important terms.
プレゼン資料で下線を使って注意を引いた。
I used underlines in the presentation slides to draw attention.
HTMLではuタグ/uで下線を引けます（最近はCSSで制御することが多い）。
In HTML you can use the <u> tag to underline text (recently it's more common to control underlines with CSS).