Offset or the et of a wheel is the measurement in mm. of the distance between the mounting (hubface) and the centreline of the wheel. This is expressed as a + or - depending on whether the centreline of the wheel is inboard or outboard of the mounting hubface.
Radial tyre numbers translate as: example 185/60x13 = tyre is 185mm wide,the height of the sidewall (profile) is 60% of the width and it fits a 13 inch wheel