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.