Items are defined in G.items You can think of each G.items entry as a recipe for an item, the definition of the item. One example:
It's of "type"/"weapon" - It's weapon type, i.e. "wtype" is "fist" - If you check G.classes - You can see which classes use "fist" weapons. The item's visible "name" is "Frozen Claw" - keep in mind that the item's keyname is "fclaw"
"grades" property determines [At what level the item becomes High grade, At what level the item becomes Rare grade], so [0,7] means it's High grade at +0 and Rare grade at +7.
Now that we know about the definition/recipe, we can move onto the actual items/products of these recipes. You can inspect your own items before we do so: character.items One example:
So actual items refer to their definitions with their "name" attributes. It's the keyname from G.items
Useful function: item_grade
Work in progress / Incomplete