G90 sets the mode of the control to Absolute.
All locations (ie dimensions) given will move the tool relative to an absolute zero, which is usually the face of the part (Z dimensions), and the center of the part (X dimensions).
Virtually all moves in your programs will be in absolute mode, so put g90 near the top of your program, in the 'header': g90g72g94f300.
(On current CNC controls, when creating a new program this 'header' is automatically created).