Let's say an NPC has flight(20). What exactly does this mean? How far can it get in a half action, charge, etc?

The flyer trait tells you to replace the characters AB with the flight modifier on the movement table to determine speed.

In the case of flyer(20) as a half action the character would be able to move at a speed of 20m. The movement table entry for such a speed if it existed on the table would look like this 20/40/60/120 mpr.

That's one fast flyer indeed. It should also be noted that some movement action must be taken each round in order to stay aloft, or the character will fall from the sky and suffer falling damage based on the character's altitude. See DH index entry for flyer for additional information.

That does indeed sound mighty powerful. Thanks for the clarification.