Many, many movie fans have seen the long, drawn-out theories about the film in general, and that's not even touching what's inside the briefcase. A popular theory says it's Marsellus Wallace's soul, while others claim that it's simply whatever you want it to be.
According to Tarantino, it's just a MacGuffin, something to drive the plot along. Meaning that it's not important, so really, we shouldn't even be talking about this.
What the Script Says:
But Tarantino's co-writer, Roger Avary, says that when they wrote the script for Pulp Fiction, it was diamonds.
Not soul-capturing diamonds. Not a giant diamond that shoots lasers. Just plain old diamonds. Why didn't they just show that? Because Tarantino had just used a case of diamonds as a major plot device in Reservoir Dogs, and he and Avary agreed that it was "too boring and predictable" to do that again. So they decided not to show them, thus giving the whole thing an air of mystery.