Can someone explain them, do you need a different cassette or a spacer or do they need different dropouts?

Most frames are 135mm spaced.

So you either need a frame specced with 142mm wide dropouts. Or some frames have interchangeable dropouts that vary in thickness to accomodate the extra width.

The freehub will be the same width. It just allows for less dishing in the wheel.

It’s just a variant of the 135mm maxle type system. Infact, it’s how maxle should have been designed in the first place. The extra 7mm is 2 x 3.5mm slots out of each dropout, so the wheel slots into the dropout. I think there are slightly different systems, certainly one by Syntace (also used by DT Swiss) and possibly Shimano and/or others. So, if you don’t already have it, you will need different dropouts.

With regards the wheel, the only difference is you need the hub conversion kit for 142mm x 12 (assuming that your wheel hub is adaptable).
Syntace X-12 system explained here, with list of compatable wheels and hubs.