if you want a really in depth explanation, just google it.
basically adj cam gears let you adjust the cam timing in relation to the crank. also if its a dual cam engine, adjusting the intake and exhaust cams independently is beneficial. on an internally stock engine there are a few hp to be made with cam timing, but on a built and/or boosted engine adjusting the cam timing on the dyno could net easily net 10whp.
adjustable cam gears must be adjusted and set while on the dyno by a tuner that knows what they are doing.