Though it is never answered in the movie what is in the ブリーフケース (just knowing that what is in there is valuable and emits some sort of golden glow), there are several theories. One is that the ブリーフケース contains the diamonds from "Reservoir Dogs". Another is that the ブリーフケース contains Marcellus Wallace's soul. I personally think that Quentin Tarantino left the idea of what's in the ブリーフケース for the viewer to guess.
The whole idea behind the suitcase is that it is the most valuable thing that the viewer can think of. Whether あなた want to think it is full of money または the Koh-I-Noor diamond, it's your choice.
