Emacs has extensive documentation built in. To look up help for a certain keyboard command, use C-h k and then the keyboard shortcut. For example to look up the help for C-x C-s you would use C-h k C-x C-s and you will see a help buffer open which documents the command and has a link to the section of code where the command is defined.

Similarly if you want to look up the help for a specific function, use C-h f function-name.