I heard about interrail about two months ago and I'm dying to try it out because it seems ridiculously cheap.

I have read something and got confused. It was "InterRailers are not permitted to travel with their pass in their own country of residence"

I don't live in a big city which means I would have to take the train to let's say dusseldorf and maybe I have to take another train in my country, say frankfurt and from there to paris (france interrail pass). Does that mean I have to buy a train ticket in germany to frankfurt and can only use the pass from frankfurt to paris and the same thing from paris to dusseldorf?

As you all can see, I'm very confused because that would mean it's not that cheap overall.