to draw a line under a word, especially in order to show its importance

All the technical words have been underlined in red. 所有术语的下面都画了红线。

to emphasize

She gave some shocking examples to underline the seriousness of the situation. 她举了一些令人震惊的例子以强调形势的严峻。

To underline their disgust, the crowd started throwing bottles at the stage. 为了宣泄他们的厌恶，观众们开始朝舞台上扔瓶子。