Chainring offset is the distance measured from where the chainring bolts onto the motor to where the the chainring's teeth sit.  The larger the number, the closer your chainline will be to the center of the bike .  Both the BBS02 and BBSHD move the chain line away from the center of the bike significantly, which can cause issues like poor shifting of excessive wear on teeth when using the lowest gears.  Chainring offset can negate some of these issues.