What is the meaning of the words with underline in a man page?
Most underlined words in man pages are for emphasis. Its is not a hyperlink .
also from man man page
italic text replace with appropriate argument.
[-abc] any or all arguments within [ ] are optional.
-a|-b options delimited by | cannot be used together.
argument ... argument is repeatable.
[expression] ... entire expression within [ ] is repeatable.
Exact rendering may vary depending on the output device. For instance,
man will usually not be able to render italics when running in a termi‐
nal, and will typically use underlined or coloured text instead.