G21 means "all moves starting now have their units specified in millimeters ( metric units )".
G21's evil twin is G20, which means "all moves starting now have their units specified in inches ( imperial units )".
An inch is 25.4 millimeters.
G21 is the default when Smoothie starts.
The command is used as such :
Which means : units are now metric until another G-code changes it ( modal command )
These resources are used as references for Gcode :