G10 means "retract the filament from the hotend using the extruder"
By default, most slicing programs will use G1 commands to retract, but this is the more modern way of doing it.
G10's evil twin is G11, which means "unretract the filament from the hotend using the extruder".
G10 means "retract" for 3D printers, but it also means other things for CNC mills.
This conflict is because the Reprap project is stupid and chose it's Gcode without any consideration for the Gcode standard that had existed for decades, and this despite the fact that the real G10 could have been useful for 3D printers.
Smoothie supports both formats : if you use G10 without parameters it will do the 3D printer thing, and if you use it with parameters it will do the CNC mill thing.
The command is used as such :
These resources are used as references for Gcode :