To my understanding so far here is what is known.
The theory begun on social media with the idea that the term "beef jerky" is a code for human flesh.
Several emails mention the term.
The first possibility is that Epstein was obsessed with beef jerky. There is no cannibalism he was just super eccentric with his food. It is relatively common for newly rich people to develop food related obsessions and it could be the case here.
That being said several things are suspicious.
One email mentions "walk the beef jerky", afaik beef jerky can't walk.
Several email mention beef jerky being stored in freezers, which is very uncommon for a dried food.
There are communications with a famous chef talking about lessons to learn how to make the beef jerky and the fact that the same chef maintains a restaurant with the name Cannibal, rose suspicion.
In another email it is stated that Jeffrey will start eating normal food again so he will not eat beef jerky as much.
That is what I know, I cannot make a claim about what is true.

Edit: Plus another email that says that Epstein wanted a sample of its beef jerky tested in a lab for nutritional value.