This is what a slatted rack looks like:
It goes on the hive below the bottom box and forms the top of the entrance above the bottom board.
But in the hottest weather, even with a slatted rack, the bees also are sent outside to help keep the hive cool both by taking their hot little bodies out of the cumulative body heat inside the hive and by working their wings.
This video was made when the temperature in Atlanta was in the 90s one recent afternoon. It's in slow motion so you can see their wings.