Flavors are groups of special abilities the GM and players can use to alter a character type to make it more to their liking or more appropriate to the genre or setting. For example, if a player wants to create a magic-using thief character, she could play an Adept with stealth flavoring. In a science fiction setting, a Warrior might also have knowledge of machinery, so the character could be flavored with technology. At a given tier, abilities from a flavor are traded one for one with standard abilities from a type. So to add the Danger Sense stealth flavor ability to a Warrior, something else—perhaps Bash—must be sacrificed. Now that character can choose Danger Sense as they would any other first-tier warrior ability, but they can never choose Bash.
The GM should always be involved in flavoring a type. For example, they might know that for their science fiction game, they want a type called a “Glam,” which is a Speaker flavored with certain technology abilities—specifically those that make the character a flamboyant starship pilot. Thus, they exchange the first-tier abilities Spin Identity and Inspire Aggression for the technology flavor abilities Datajack and Tech Skills so the character can plug into the ship directly and can take piloting and computers as skills.
In the end, flavor is mostly a tool for the GM to easily create campaign-specific types by making a few slight alterations to the four base types. Although players may wish to use flavors to get the characters they want, remember that they can also shape their PCs with descriptors and foci very nicely.
The flavors available are stealth, technology, magic, combat, and skills and knowledge.
The full description for each listed ability can be found in Abilities, which also contains descriptions for type and focus abilities in a single vast catalog.
Unless otherwise noted, you cannot choose the same ability twice, even if you get it from both your type and a flavor.