Flaws are "anti-feats", you can choose up to two at 1st level, and for each one you take you can take an extra feat at 1st level.
They tend to have slightly stronger negatives than feats give as positives (imagine a feat that gives bonuses to something, now turn that bonus into a penalty and increase the penalty by 1/3), but there is room for min-maxing as you weaken things your character probably won't do as you take extra feats. They even have a "Behind the Curtain" sidebar in the section advising caution in game balance with them and in creating new ones.
Some examples of flaws:
Poor Reflexes: -3 to Reflex Saves
Unreactive: -6 to Initiative checks
Inattentive: -3 to Listen and Spot checks
Personally, I don't use them in my game because my entire group took one look at them and agreed that there was way too much min-max potential there. There is an alternate system in UA called "Traits" which is a related set of flaws and advantages as a joined set, like
Plucky: +1 to Will Saves, -1 to Fort saves
Nearsighted: +1 to Search, -1 to Spot
Honest: +1 to Diplomacy, -1 to Bluff