As a not-native english speaker, I sometimes bump into topics that conceptually are rather easy, but in fact much harder (for me) to grasp than they ought to be, because for a native english speaker they are probably self-explanatory, and therefore are hardly ever explained in detail. On such terms is domain, which took me a while to understand. So let me try to come up with a detailed but easy explanation…

A domain is the area of the business you programm for, so if, for example, you work on a software for a car manufacturer, your domain might be “cars” – only, of course, if the topic of the software is also related to cars! Your domain might just as well be human resources related, if for example, working for the exact same car company, you are developing a software for timekeeping.