The higher the better.

Although they're not directly related to weight ratings you can actually use them that way. Simply put a decimal point in there so that the 400 becomes 40.0, 575 becomes 57.5, 640 becomes 64.0 etc etc. That will give you a pretty good guide about the relative strengths of the horses.

I think the ratings are based on lengths beaten. I can't remember seeing a negative rating, but I've seen some very low ones and it simply means that the horse had a real shocker that day and was beaten a long way. If a horse has one rating which is much lower than all the rest, it's probably best to just ignore that rating.