What does it mean to have an overall pet rating of 100??

It means that the combined stats of one of your pets - its level, strength, defence, health and speed (Intelligence doesn't count for the Blue Building) - need to add up to at least 100.

You can train them and level them up here at the training center in Riverside Valley.

It has to do with the battle stats of your Pets, it has always been Level + Strenght + Defense + Health of one of your Pets = 100 now where a new Speed stat was added a few days ago I don't know if it's included as well. Intelligence however doesn't count. C:

okay, thank you so much guys! :D I'm trying to get into the blue building. ^^

Speed doesn't count towards your pet's rating for the Blue Building, fyi.