G1 causes the machine to travel in a straight line with the benefit of a programmed feed rate (using "F" and the desired feedrate). This is used for actual machining and contouring.
Example:
N120 Z0.1 F6.0 --move the tool down to Z=0.1 at a rate of 6 inches/minute
N130 Z-.125 F3.0 --move tool into the workpiece at 3 inches/minute
N140 X2.5 F8.0 --move the table, so that the spindle travels to X=2.5 at a rate of 8 inches/minute