its simple... you take a wheel of a certain width, say a 7x15, then fit narrower tyres to it... something like a 175/50 is the usual on a 7" wheel.
ive got 215's on my 8.5" wide brocks, but will be going for 205's when i replace tyres cause i want them a lower profile and a bit more stretched.
there is a limit before it starts getting unsafe, if the tyres are too narrow you run the risk of the tyre popping off, but then saying that... ive never heard of it happening, and know someone who has just fitted 165/55/15 tyres to a 7x15 wheel, now they are stretched.