OK, i know that the whole idea is around the suitcase having "Pulp Fiction" in it, but what is it really?

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