The HTML u element is used to create underlined text.
Traditionally the u element has been surrounded by a great deal of controversy, and it has been used different purposes by different authors.
The WHATWG currently states the following in their Living Standard:
The u element represents a span of text with an unarticulated, though explicitly rendered, non-textual annotation, such as labeling the text as being a proper name in Chinese text (a Chinese proper name mark), or labeling the text as being misspelt. – https://html.spec.whatwg.org/multipage/semantics.html#the-u-element
The best advice we can give you in regards to its use, is to avoid using it if you are unsure about why you are using it. If you are using it for presentation, you should consider using span elements instead.
HTML u Example
<p>You can <u>not</u> fly without fual for your airplane</p>
The u Tag can be used to underline text.