A lot of people don't know this... (and I myself just figured it out)
For any individual alloy or steel wheel, ET and Offset both have the same meaning and value. The term ET originates from the German word 'Einpresstiefe' which translates as 'insertion depth'.
A:The distance between the rim's center line and its
mounting surface. From the picture below it should be
obvious why it's important to retain proper offset when
you change rims: tire not centered properly affecting
drivability (negative roll radius changes), bearing load,
rubbing on the struts or wheel arches, etc.
Cross sectional view of a rim: