The correct definition of cam duration is: The amount of time, measured in degrees of crankshaft rotation, an intake or exhaust valve is held open.
Is high or low duration better? depends on what the motor is built to do.
First you need to figure out what you want the motor, to do and RPM you will be running then talk to a good builder to figure out what cam you should be using to achieve those goals. there is more to choosing a cam then duration.
Cam Specs & Effects, duration centerline separation and lift, more on pereformance engine parts
hear is a link on basic cam measurements and the what it effects.