As the title says and the thumbnail shows, this pack rebalances chest loot. This does not mean every chest has better loot! Most structures have a larger array of items that generally make sense (i.e. raw ores in underground structures), but none of the structures have been made OP by any means.

All chests also have a 20% chance of containing a music disc. Each type of structure has two music discs you can find in chests there, and each disc (except for Music Disc 5) is in at least 2 different structures.

There's also some extra minor changes to loot based on the biome the chest is in. For example, Abandoned Mineshafts have extra Glow Berries if the chest is in the Lush Caves biome.