By zone offset, we understand the amount of time needed to be added/subtracted from the GMT/UTC time in order to obtain the date-time for a specific zone on the globe (for example, Perth, Australia). Commonly, a zone offset is printed as a fixed number of hour and minutes: +02:00, -08:30, +0400, UTC+01:00, and so on.

So, in short, a zone offset is the amount of time by which a time zone differs from GMT/UTC.