So I will break it down a little bit for you so that you know why they say what they are. (not saying you're stupid, just making sure everyone is on the same page)
Offset is the difference from the center of the rim (widthwise) that the surface that meets the brake rotor is.
If the surface is closer to the outside of the rim, then the difference from the middle is a + number.
Closer to the inside of the rim is a negative number.
So if you have stock a +48 and want the rim to be sitting out more, you want more material on the rim and to bring it closer to 0. 12mm is as close to 1/2" as you're going to get, and when you subtract that from 48, you get +36 and that's a common offset. Hope this makes it easy to understand it. You kind of have to visualize it a little.