All items in the game are given a Grade when received as a drop, purchased or crafted. On the inventory screen press and hold on an item to see its grade. If you press and hold on a stack of items it will display the amount of each Grade as [D,C,B,A,S]. Once you have unlocked the Enchanter you can use them to reroll item Grade to try for better stats or a higher Grade. Grades are also given to determine how effective your Heroes were on a Quest, with the item Grades matching the Quest Grade. The F Grade only occurs when you fail a Quest, and no items will be received as a result.

From lowest to highest
ID Name
Quest grade F GB
Quest grade D GB
1 D
Quest grade C GB
2 C
Quest grade B GB
3 B
Quest grade A GB
4 A
Quest grade S GB
5 S