A question about polls

Can someone please explain for me how weighting affects poll numbers? I see on the Rasmussen site:

Like all polling firms, Rasmussen Reports weights its data to reflect the population at large (see methodology). Among other targets, Rasmussen Reports weights data by political party affiliation using a dynamic weighting process. During the final two months of Election 2008, party weighting targets are updated each Sunday (see additional information). For polling data released during the week of October 5-11, 2008, the partisan weighting targets used by Rasmussen Reports will be 39.3% Democratic, 33.3% Republican, and 27.4% unaffiliated.

Also see that, while other polls show the race tightening, Rasmussen is holding steady. And since they’re reporting by political party, I’m confused as to how it could change on a week by week basis.

If anyone is so kind as to try to explain this to me, please try and make it simple…numbers are not my strong suit.