what wheel offset

[Typhy 0]

i keep seeing this on ebay with alloy wheels ( looking at bbs rs's hmmm )

 

anyway what does this mean and what would be the desired "offset for a 1990 corrado 4 bolt setup.?

 

thanks

 

http://cgi.ebay.co.uk/3pcs-BBS-RS-wheel ... 286.c0.m14

 

these are amazing