These files are the data from the .txt files from d2_patch.mpq, as well as a couple of helpful extra files. Additional files that aren't directly based on existing .txt files are:
Atomic treasure classes that aren't explicitly listed in TreasureClassEx.txt. The numeric value for each is the rarity of the item.
A collection of the more actionable data such as weapons, armor, experience, etc. Good for bundling, but if you're only using a subset of the data it might be better to use the individual files.
A tiny style sheet that adds the AvQuest font, and an 'avquest' class that applies the font and a couple additional styles that adjusts the font to look closer to diablo 2. https://blizzhackers.github.io/d2data/css/d2font.css
Examples on how to use this data exist at https://blizzhackers.github.io/d2data/