The offset of a car's wheel is the distance in millimetres from the centreline of the wheel to the flat face of the mounting surface of the wheel.
The offset can be either a positive or negative figure. A positive offset is when the mounting surface is in front of the centreline closer to the face of the wheel, while a negative offset is when the mounting surface is the over side of the centreline deeper in the wheel. Most cars have positive offsets and deep dish wheels usually have a negative or very low positive offset.
If a wheel has an offset of 'ET45' it means it has a positive 45mm distance from the centerline of the wheel. The ET is short for Einpresstiefe which is a German word that translates as 'press depth'.