IPhone battery cases: what is the best mAh?

I'm looking to buy a battery extending case for my iPhone. I'm seeing some that say 1900 mAh, 2000mAh 2500 mAh etc. I was wondering if anyone could explain what that is and which would be the best for an iPhone 4S with AWFUL battery? (It's at 100% before I go to work. It stays in my pocket for 6 hours and is down to 3%)

Higher mAh simply means that the battery will last longer. If you need your battery to last a long time, go for something larger like a 2000 or 2500 mAh.