Civic EG

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Offset is the distance (in millimeters) from the center of the width of a wheel, to the surface of where the face of the wheel mounts to the hub.

Here's a link with a detailed explanation:
http://www.worldoneperformance.com/blog ... sets-work/

to put it simply...

it basically dictates how far in ur wheels sit... or how far they stick out.

typically the higher the number, the more inwards into the fender it goes

the lower the number, the more it will stick out of the fender.