You each get so many injury boxes, and using up those boxes by taking an injury (and therefore checking off a box just like you would if you were using an aspect) will lead to "consequences"—which are injuries that act like aspects, meaning they can be called upon.
So, if one of you is injured enough to evoke a consequence (let's say Dru gets stabbed in the hand) and he tries to ward off his attacker with a dagger and he rolls really well, I could call "you've been stabbed in that hand" and make him re-roll his score in the hopes of stabbing him again. Alternatively, you can also do this to NPCs you injure to make me reroll for your enemies. (Calling a consequence involves Fate point usage—I call yours, I give you a Fate point. You call the enemy's, you give me a Fate point.)
In our game we will also have "stress" boxes that are for the times you're beaten by 1, and these are important because they go away after each scene whereas the other injuries take longer and/or have to be healed by some sort of medicine person or Healer. Stresses also don't produce consequences that can be called.
Here's the breakdown:
You each get 5 boxes for "stress", 4 boxes for "mild" injuries when the NPC beats you by 1-2 points, 3 boxes for "moderate" injuries where you're beaten by 3-4 points (you can also choose to take two milds instead of one moderate and so on), 2 boxes for "severe" where you're beaten by 5-6 points and 1 box for "extreme" where you get your ass handed to you and if all your boxes are filled and you take another hit, you're dead. If you wish to remain conscious after all your boxes are filled you will have to roll for it.
There are also temporary consequences, so if I want to kidnap you to torture you later NPCs can KO you, or vice versa you can KO my NPCs if you roll well enough.
Like temporary, there are also escalating consequences. They will get worse if you don't attend to them. (Ross' example is if I set one of you on fire. It'll start as a mild as your sleeve catches, but go up in severity if you ignore it. You may, in fact, die of a cock fire if you ignore this problem.)
Stresses—go away after the scene (basically represent things like mild cuts and abrasions etc. that can be ignored)
Mild—takes a bit of time, but will heal after a good rest.
Moderate—you need some healing or a few days rest before this will go away, and it will fall down into "mild" as it heals throughout time.
Severe—you need some serious healing, and some serious bed rest time to let your wounds fall back down to stresses.
Extreme—one more scratch and you're dead. Get thee to a Healer! (Or lay in bed for months.)