Not quite understanding automatic cam adjustment (2003 Pathfinder)?

I have a 2003 Pathfinder. The mechanic discussed with me that there is some sort of hydraulic cam adjuster that is having trouble. I believe the cam basically rotates and has bumps on it that work a lever that open and closes the valves (affects timing of the opening and closing of valves). He said that this cam adjuster is hydraulically controlled by the oil in the car (use normal engine oil?). I also think he mentioned something about the cam adjuster controls the index, which I was interested in roughly how this works. It seems to affect the horsepower and perhaps the timing. Thanks for any insights and corrections in my understanding (autoshop was many years ago).