Resistance is just one measure used to denote the "hotness" of a pickup. It's a rather arbitrary measure and really only relates to the number of winds. Many pickup manufacturers use this measure though, mainly as a relative "hotness" scale. If you want to achieve a similar level to a known pickup (seymour duncan or whatever) then the quoted resistance of the pickup is one measure to get you in the ballpark (all other things being equal).
This is my understanding, having never ever wound a single pickup in my life.
cheers,
Gav.