G91 means “all moves starting now are in relative coordinates”.
This means positions are given relative to the current position, and not relative to the 0,0,0 point of the current workspace coordinate system ( which is what G90 does ).
When Smoothie starts, G90 is the default mode, not G91. G91 is often used when G-code is manually sent to the machine.
G91's evil twin is G90.
The command is used as such :
Which means : starting now all positions are relative.