The “section”[0] or “subpage.” Programs are section 1, hence ‘dd(1)’ The idea is that it can be possible for a library function (section 3) to have the same name as an executable. In which case, you’d type into your shell:

    man 1 <name> # executable: <name>(1)
    man 3 <name> # function:   <name>(3)

Without the distinction argument, man will throw its arms up in the air and give up.

However, if there’s no ambiguity (such as with ‘dd’ where only the executable exists), you can drop the section number parameter when running man. man will then search all the sections, see that ‘dd’ only exists in section 1, and go from there.

[0]: https://linux.die.net/man/