A power cost adjustment tracker is a mechanism that follows or "tracks" some varying costs that a utility might incur in providing service to consumers.
What are these unpredictable costs? Utilities use trackers for various costs, including fuel, environmental requirements and purchased power above estimated levels that are projected for a given period. If costs fall, the tracker declines and can even become negative, so you would see a bill credit. The tracker amount is multiplied times the kilowatt-hours used.