If you tune your ADF (automatic direction finder?) into an NDB then the arrow points to where the beacon is in relation to the aircraft, sort of like a compass. You can make non-precision approaches with them, or, enroute when there's none around worth monitoring, make good use of the equipment anyhow by tuning in regular radio stations and turning up the ident.
I was taught all this at some point in my PPL training aswell. I agree MS FlightSim is good for familiarising with instrument navigation, saved a half hour off my PPL hours, so it pays for itself. Just don't practice anything else, like landings!