What does it mean "See something(3) or see something-else(5), etc..." in the man of a command in terminal?

I have a simple question I made my self but didn't find any answer and hints.

When I type for the man entries of a command, e.g. "man git", I get the following:

...

DESCRIPTION
       ...

       See gittutorial(7) to get started, then see Everyday Git[1] ...
       The Git User's Manual[2] has a more in-depth
       introduction.

       After you mastered the basic concepts, you can come back to this page to learn what commands Git offers. You can learn more about individual Git
       commands with "git help command". gitcli(7) manual page gives you an overview of the command line command syntax.

       Formatted and hyperlinked version of the latest Git documentation can be viewed at http://git-htmldocs.googlecode.com/git/git.html.

Could someone explain what are those "See gittutorial(7)...Everyday Git[1]...gitcli(7)" etc.? I mean, I know that they are telling "look at this resource if you want to know the basic concepts, for more info, etc.", but actually how should one interpret it? What is the meaning of the numbers (like 7, 1, 2, etc..) inside the parenthesis or brackets? And where I can find the resources the manual is telling me to see, do I have to type something in the man prompt, or search on the Internet?

I just would like to ask for an elucidation.

Thank you for your attention!

Solution

Man pages are splitted into sections

to access a manual page inside a specific section you prepend the section number before the manpage name, for example try:

man 3 fork

man 2 fork

Usually the square braketed numbers are links to additional material in the NOTES section of the manpage (scroll to the bottom)