It's offsets from the program line. When you're cuttining in a straight line (G01) is G41 offset to the left of the program line and G42 offset to the right of the program line. You must use D in conjunction with the G41 & G42 as the D value sets the amount of the offset. Eg: G41 G01 X10.0 Y25.0 D0.15, this translates to a offset of 0.15mm to the left of the program line.
Am I clear? If not Pm me and i'll e-mail you an example of a program.