The git(1) manual page says:
See gittutorial(7) to get started, then see Everyday Git for a useful minimum set of commands. The Git User’s Manual has a more in-depth introduction.
Where is either "Everyday Git" or "Git User's Manual"?
The links can be found at the very end of the man page:
1. Git User’s Manual file:///usr/local/share/doc/git/user-manual.html 2. Git concepts chapter of the user-manual file:///usr/local/share/doc/git/user-manual.html#git-concepts 3. howto file:///usr/local/share/doc/git/howto-index.html 4. Git API documentation file:///usr/local/share/doc/git/technical/api-index.html 5. email@example.com mailto:firstname.lastname@example.org
[number] notation usually means that it references some document, and at the end of (page,document,...) you can find location of these documents. I encourage you to look around more before asking the question ;)