Welcome to one of the most Googled props in cinematic history.
Even decades years later, people are still debating what's in the briefcase. We've all seen the astonishment of Jules and Ringo as they stare into its glowing content…so…what is it?
Answer: two battery packs and a lightbulb.
Okay, fine. Let's suspend disbelief for a second. This is a movie, after all, and what you don't see isn't actually there. So let's talk about what could be in the briefcase, with the help of a few popular fan theories.
The first possibility is that it contains diamonds (they're pretty shiny, right?). This was actually the first idea that writers Tarantino and Avary had—they were probably supposed to be the same diamonds from Reservoir Dogs. But then the writers thought diamonds was a bit cliché, so that idea went out the window (source).
Possibility #2? Some people have suggested it's the gold Elvis suit from the Tarantino/Avary film True Romance. Again, a good reason for it to be glowing yellow, and we all know Tarantino likes referencing his own movies in his other movies.
And finally, the fan favorite: the briefcase contains Marsellus Wallace's soul.
Yep. Marsellus has sold his soul to the devil, and now he is trying to get it back.
You probably want some evidence. Sure thing. For starters, we see in Brett's apartment that the combination of the briefcase lock is 666—the Biblical number of the devil. That would also explain the divine intervention that Jules witnesses, because of course God would intervene to help someone rescue their soul from the devil's grasp. And finally, we have the Band-Aid on the back of Marsellus' head. That's is the spot where the devil extracts your soul from your body according to…um, according to…well, no one aside from random online people writing on forums (source).
Ultimately, the briefcase is really a MacGuffin —and a pretty good one, considering all the theories it's generated. A MacGuffin, a term popularized by director Alfred Hitchcock and often used in a derogatory way to describe a cheap tactic in storytelling, is an object that the characters of a story pursue in order to drive the plot forward. Essentially, its significance is of a driving force and really has no importance inside the narrative itself.
In this case, a MacGuffin is exactly what Tarantino wanted. In interviews, he's confirmed that inside the briefcase is "whatever the audience wants it to be" (source).
We're going with a gold-leaf Kahuna Burger.