An impairment is a general action penalty. Impairment reduces your ActionValue in each skill, but never below the default value.
Impairment is not the same as an ActionValue penalty. It is generally much worse, as it affects your defenses and in some cases reduces the effect of your skill-value dependent powers.
The most common source of Impairment is WoundPoints, but there are many other sources as well.
Impairments are always a penalty. There is no such thing as positive or negative Impairment. A -1 Impairment, a 1 point Impairment, and a Impairment 1 are all the same thing.
Impairments from different sources generally do not stack, only the worst applies.
Examples
Wound Points
Wound Points accumulate from attack to attack. When you reach 25 Wound Points, you are severely hurt, look like shit and take -1 Impairment. At 30 Wound Points, the trouble gets worse - you look like you are ready for the morgue, and only willpower keeps you going. You suffer a -2 Impairment in high-motivational situations like combat, but when the tempo slows down, so do you. Rest is generally your highest priority. Beyond 30 Wound points, you are on the brink of death and start to take DeathTests.
Restraint
When you are physically restrained, you take a penalty to most actions. Physical actions are penalized for obvious reasons, most social interaction is penalized because of your obvious disadvantage, and even mental and or technical tasks usually require freedom of movement to complete successfully. The GM may choose not to appy this impariment to certain taks, such as a Seduction stunt based on your state of helplessness.
For each limb that is bound, you take a one point of impariment. This assumes you still have some mobility, such as being handcuffed. If the limb is completely fixed, such as being nailed to the floor, the impariment is two points.
Darkness
Darkness causes three kinds of penalties. First, it causes two points of general Impairment because you are blind. Second, it penalises all targeted actions by -2 because you cannot see what you are striking at. Third, it impoises a range penalty of -1 per meter as you try to blindly aim a ranged attack.
Of course, this is only complete darkenss or blindness. In most cases, there will be some light, even if it is faint. This removes all penalties except the range penalty. Numerous schticks will mitigate darkness penalties, often with some range limitations. Finally, note that "blind" in this context means unable to use your primary sense - many dogs are more hampered by losing their scent rather than their sight.