Here is a link to the WotC article about bounded accuracy, in other words, what they have been referring to as flatter math.
This was another topic of brief discussion Saturday after the playtest and the group seems to feel that it is going to prove itself to be a welcome innovation for 5e.
Mearls covers the details well in the article so there is not much point to go into it here.
After the playtest I can visualize the characters entering a large circular room with high ceiling and a balcony with the intent of facing a villain. They also find themselves ringed by a couple dozen dagger weilding kobolds ready to throw at a signal and they would know this isn't a good thing. Sure the kobolds are only 2 hitpoint wonders, but they outnumber the group by a lot and they are murder with those thrown daggers.
I think I'm inclined to like the flatter math.