These two drawings show the difference between a straight neck and one set up with arc relief to allow room for the vibratiing string. The lower drawing is exaggerated slightly as in reality the relief measured at the middle of the neck should only be about 0.25mm
Straight neck - gap between frets and string continually increase by linear increment
With arc relief - gap between frets and string opens intitially but begins to level out towards the 12th fret then increases little, becoming almost parallel