From DriveThruRPG for the Inner Core Rules Item:
Adventure in any world setting, any genre, as simple one shots or extended campaigns.
This is the beginner's minimum for playing an easy entry, low preparation role playing game, where a game master presents a setting of their own devising, and players run their characters through the setting on whatever adventure arises.
Some actions are resolved with dice using the Dice Engine, the only formal structure in the inner core of the All Us Gamers rules. The dice are d20s in a pool that can give results from 0 (a disaster) to 5 (an incredible success). Character aptitude, their focus points and action difficulty determine how it plays out. This is used for both actions and saves.
The players and game master cooperate and use common understanding to pull the game together in the inner core, which is light but nevertheless complete.
Once you have mastered the inner core (it shouldn't take long) then watch out for the outer core and world setting modules.
The outer core builds on the inner core to allow richer characters that can grow in ability, more detailed but flexible conflict resolution, and specific action areas of interest like stealth, investigation, diplomacy, network hacking and beyond.
World settings will take your play further with game structures to help adventures flow well.
Get yourself prepared with the inner core, and if you want to you can stick with it without the extras, or go to the richer outer core detail when you get to the world settings.