If someone attacks you you get a free shot at them where you are able to do some damage to them without response. Only works on defense. Like if someone were charging you with an axe, you get one shot from your bow as they are running down the hill toward you.
There are two types of first strikes first strike CHANCE (Drill I and III), in which your unit when defending has a 50% chance to do a first strike and first strike (Drill II and IV) in which your unit when defending has a 100% chance of getting a first strike.