If you create a file on your SD card named on_boot.gcode, the contents of that file will be executed every time the machine boots up.
This is useful if you need to set things up a certain way, or perform actions, every time the machine is reset.
Here are some useful G-codes you might want to use in your on_boot.gcode:
G28 (3D printer mode) or $H (CNC mode), instructs the machine to seek the endstops on all axes, useful if you want your machine to always know where it is. Without this, features such as soft endstops will not work until the user manually issues an endstop seeking (home) command.G92 To set what the current position isG0 To move the head somewhere in the work areaG91 or G90 to set the current mode to absolute or relativeG53 G0 to move to a position in absolute machine coordinates instead of the current workspace coordinate systemM105 to set a given temperature on a heater so it starts pre-heatingM106 to turn a fan on at boot timeSee supported-g-codes for more information on these and other G-codes.