I am not quite understanding what this is, could someone explain this to me
This is the rules that are used to produce the player ratings. It helps you fine-tune how players are listed - i.e, if you want to be a more pass-oriented offense, you might value pass blocking above run blocking, and would prefer that a player with 75 pass block and 25 run block have a rating that is higher than someone who wants to have a more run-oriented offense would. So you might set your pass block attribute to stay at 100, but might lower your run block attribute to 0, which would mean that instead of the player being rated at "50" (where both pass block and run block are equal) he would be rated at "75." This only affects your view of the ratings, and is used in the search result lists as well as for any decisions the AI makes on your behalf.
You usually will leave these close to the defaults, but the idea is to give you complete control over how you want your players' ratings to sort.