To 'underline' means to draw a line under a word especially in order to highlight its importance. To 'underline' something can also mean to emphasise it. You will often hear expressions such as 'I can't underline the seriousness of this' or 'I can't underline this point enough'. This is a popular expression used often in professional contexts. Similar in meaning to 'stress this' 'to highlight' or 'to emphasise'. 

1. In his speech, the politician was quick to underline his commitment to fighting climate change. 
2. The fact that the hospital kept Tom in for treatment underlined the seriousness of his condition.

1. Stephanie's large list of contacts and ability to seal a deal underlines her importance to the negotiating team.
2. Last month saw more heavy losses, underlining our urgent need to modernize the business and bring it more into line with our competitors.   

To 'underline' means to draw a line under a word especially in order to highlight its importance. Similar in meaning to 'stress this' 'to highlight' or 'to emphasise'. 

1. The key points of the text were underlined in red so that they stood out more clearly on the page. 
2. I was asked to underline all of the key points in the text so nobody would miss them.