a cam opens and closes the valves in the heads,via lifter,push rod,rocker. How far the valve opens is called valve lift. (how far the valve is lifted off the valve seat) duration is how long the valve is lifted from the seat(time wise) counted in degrees of crankshaft rotation. A long duration cam keeps the valves open for a longer time to allow for more air flow,higher lift cams make the opening bigger,to also allow more air flow.
you need a cam that allows the exact correct amount for your engine and application