The HTML <u> element represents a span of inline text which should be stylistically differentiated from normal text, due to its typographical semantics. This element was deprecated in HTML4 but then reinstated in HTML5.

The <u> tag is used to underline text. However, because underlined text is also the standard way to represent hyperlinks, using the <u> element can potentially confuse users. For this reason, it’s often better to use CSS properties for text decoration, unless there’s a specific typographic convention that justifies the use of the <u> tag.

Here’s an example of how the <u> element can be used:

<p>I want to <u>underline</u> this word.</p>

This will render as: “I want to underline this word.” The word “underline” will appear underlined.

In HTML5, the <u> element is redefined to represent 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.

But in practice, the <u> element is used less and less, as it is recommended to use CSS (text-decoration: underline;) to underline text. This is due to accessibility concerns, as the underline can be confused with a hyperlink, which is the default browser style.

In summary, while the <u> element has its uses, it’s generally better to handle text decoration through CSS. This provides a greater degree of control over the appearance of the text and helps avoid potential user confusion.