Though it is never answered in the movie what is in the портфель (just knowing that what is in there is valuable and emits some sort of golden glow), there are several theories. One is that the портфель contains the diamonds from "Reservoir Dogs". Another is that the портфель contains Marcellus Wallace's soul. I personally think that Quentin Tarantino left the idea of what's in the портфель 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 Ты want to think it is full of money или the Koh-I-Noor diamond, it's your choice.
