<u> text </u>tells the browser to render the text with underlining. The HTML standard defines this as a physical style as distinct from logical styles. There is no logical equivalent.
This is <u> style
The meaning is that the physical style directs the browser to display in a particular typeface whereas the logical style directs the browser to display with a distinctive emphasis leaving it to the browser to decide how this is achieved (underlining or distinctive colour might be used).
Note. Many users set their browsers to underline link text as well as or as an alternative to displaying it a distinctive colour. The use of the <u> tag may cause confusion to such users.