If a wheel is 8" wide (8j) and has an offset of 0 (et0) it means the mating face of the wheel (the face that touches the hub) is exactly in the middle of the wheel. As soon as you have a positive offset, et40 for example, it means the mating face is move out 40mm towards the outside of the wheel. This in turn means the wheel will go 40mm further into the car. And vice versa with minus (et-8 for example) offsets.
So if you know your existing wheels are 7j with an offset of et35 and you want you new rims to stick out a further 10mm, you want either 7j et25...
Or you can start getting more complicated and do some maths to consider different widths. For example you could go for an 8j rim (25.4mm wider). If it has exactly the same offset it means it'll be 12.7mm wider in both directions from the hub. So if you still only wanted 10mm more and not 12.7mm you'd need an 8j et27.7 (et28).
To make it easier to understand...
