G28 means "seek the endstops for each axis, stop once they are found and this position is now the origin for the machine".
This G-code is specific to Reprap and does not conform to the normal G-code standard.
The command is used as such :
G28 X10 Y10
Which means : move until you hit the X and Y endstops, then set the current X position to 10 and the Y position to 10
You can also use the Gcode without any parameters :
Which is equivalent to doing :
G28 X0 Y0 Z0
|X||Move until you hit this endstop then set the position in this axis to this value||G28 X0|
|Y||Move until you hit this endstop then set the position in this axis to this value||G28 Y0|
|Z||Move until you hit this endstop then set the position in this axis to this value||G28 Z0|
See the Endstops module documentation.
This page assumes you are using a cartesian machine.
If you are using a Delta machine, then you can not individually home axes.
Instead, all "actuators" are homed together to the top of the machine, which sets you at the top of the Z work area, and at the center of the XY work area.
These resources are used as references for Gcode :