Root is an active generator program which, when activated, generates +6 PWR, but then takes -1 PWR per turn for the next 6 turns while on cooldown. Root can be bought from some Server Terminals for 300 credits.
Root is quite a difficult program to use well, and even then it is still not a very good program.
The simplest way to use Root is for a quick boost of PWR in an emergency; but in that case it would normally be better to have a different generator to avoid those emergencies in the first place.
Because Root takes away the same amount of PWR in total that it generates, to get a net gain of PWR you need to start your turns with 0 PWR while it is on cooldown. This means Root can only serve the role of a generator in low-PWR strategies. Even then, Root only generates at most +6 PWR every 6 turns, the same average PWR per turn as Power Drip, but much harder to manage.