Simply: Manual Pages. These are "manuals" that come with most Linux software packages. They explain how to use the software or command.
The man pages can be accesses by opening a terminal session and typing "man [software or command]", so if I wanted to learn more about samba, I'd type "man samba". This will display the samba manual a little bit at a time. To proceed reading, press "Enter" to advance one line at a time or "Page Down" & "Page Up" to advance or go back a page at a time. Press Ctrl+z to exit the manual. The man pages can also be accessed through the SUSE Help Center.
Not all software packages or commands will have a manual, but most will. New users may not get a lot from the man pages at first because they are written in a style that may be over their heads, but they will come in handy as you become more experienced with Linux.
This site explains more about man pages.