You are reading the System Reference Document for Sufficiently Advanced - a condensed version of the rules that can be found in the Second Edition corebook. This section teaches you the rules of the game. The other sections are the Character Creation and Major Technology sections, which teach how to create a character and describe the most influential technologies. You can also find many terms defined in the Glossary.
If you are new to the game, you should start with Complications and Advantages. You will also use Basic Actions, Conflict, and Theme Use activation in every game session, so they are an excellent place to go next.