V-T If one thing, for example an action or an event, underlines another, it draws attention to it and emphasizes its importance. 突出显示; 强调
V-T If you underline something such as a word or a sentence, you draw a line underneath it in order to make people notice it or to give it extra importance. 在…下面划线
Write this down and underline it.
I do not underline my optimism, but It just is so.
And, secondly, we need to underline the need for the government to take leadership on these issues.