All subatomic particles have a feature that physicists call "spin." This kind of spin is not like the rotation of a top or a planet, even though it uses the same name. It's actually a specific kind of angular momentum, or combination of the particle's velocity and mass. I will confess to not fully understanding this myself. You may now confess your lack of surprise at my confession.
In any event, this special kind of spin can be measured, and every kind of subatomic particle has its own specific spin. So all neutrons have a certain spin, all protons have a certain spin, and so on. When physicists began to discover that subatomic particles -- so-called because it was thought that they were the building blocks of atoms, which were themselves known to be the building blocks of matter -- were themselves made up of smaller things eventually called "quarks," and that quarks themselves also had measurable and specific spin, then the most likely relationship was that the spin of the quarks, added together, created the spin of the particle in which they were found.
Nuh-uh. In fact, when physicists measured the spin of the quarks and performed that calculation, they did not add up to the known spin of a proton or neutron. Writer Edwin Cartlidge, in an article in Physics World, said this problem was sometimes called a "spin crisis." The Obama administration press office briefly perked up its ears at that before realizing that the term did not mean what they thought it meant.
So, physicists began to wonder if particles called "gluons" might be a part of the answer. What kind of spin do gluons have, and does it combine with the quark spin to produce the spin of a proton or neutron? Believe it or not, this is a question more easily asked than answered. But after examining data from the Relativistic Heavy Ion Collider at Brookhaven Laboratory -- which Cartlidge is cool enough to call a "gluon gun" -- physicists were able to come up with an answer. Does gluon spin plus quark spin produce proton or neutron spin?
Maybe. Which may not sound like much of an answer, but when you compare it to the previous answer physicists were having to give -- "We dunno" -- it's definite progress.