On a CNC mill, G28 means "go to the recorded origin point in the machine coordinates workspace".
The machine coordinates are the position of the machine relative to the point at which the endstops are hit.
This is by opposition to the workspace coordinates, which are the coordinates you use to position a job you want to do.
The origin point is a recorded position, it is arbitrary and you choose it, but it is generally the point you want the tool to be at when it is not doing anything. It is not necessarily the 0,0,0 point of the workspace coordinates, but it can be if you want it to.
The command is used as such :
Which means : to the origin position.
Note : This G-code currently only works for the X and Y axes, but ignores the Z axis. This might change in the future, if you are not sure, just try it out manually on your machine.
The G28.1 Gcode is used to set the origin position for the machine.
See the endstops module
The speed at which the movement to the origin point will occur is the "seek" speed, which is the speed of the last G0 command issued, or if none was ever issued, the value of the default_seek_rate configuration option.
If you are looking for a way to "home" to the endstops, the same way G28 does on 3D printers, use the $H command to trigger this behavior, and configure it using the Endstop module.
These resources are used as references for Gcode :