Spun Out of Proportion: The Proton Spin Crisis (2017)

Yes, essentially it’s energy held by gluons and quarks inside the proton itself. What’s meant by “binding energy” is that it’s not the total mass of quarks and gluons inside the proton but their “field energy” which in QFT terms is, I imagine, their field tensor contributions and in heuristic terms is like the energy stored in a compressed spring, only the “springs” in question are gluons within the proton.