This command is Canned Cycle used for drilling cycle.
G81 followed by axis, R & F value.
X, Y, Z, A, B Value - specifies the axis you wish to move following the distance to move. The distance value will be the current machine units in use.
R Value - This is the retract position in the Z axis. This value must be specified.
F Value - The F value defines the feedrate at which the axis will move at.
N10 G99 G81 X10 Y10 Z-8 R2 F100N20 X20N30 X30
The first line moves the X & Y axis to 10mm position with retract plane set to 2mm, drilling to Z -8mm at 100 mm/minute feedrate and starts drilling.
The second line moves X axis to 20mm position and drill a hole as per the same values.
The third line moves X axis to 30mm position and drill a hole as per the same values.