G90 means “all moves starting now are in absolute coordinates”.
This means positions are given relative to the 0,0,0 point of the current workspace coordinate system, and not relative to the current position ( which is what G91 does ).
When Smoothie starts, G90 is the default mode. However it is common to still add the command at the beginning of G-code files.
G90's evil twin is G91.
The command is used as such :
Which means : starting now all positions are absolute.