It explains the reason why we don't believe that the proton spin is due just to the 3 quarks that make up the proton, and in the process, you get an idea how complicated things can be inside a proton.
There are three good reasons that these three components might not add up so simply.Expect the same with a neutron.
- The quarks aren't free, but are bound together inside a small structure: the proton. Confining an object can shift its spin, and all three quarks are very much confined.
- There are gluons inside, and gluons spin, too. The gluon spin can effectively "screen" the quark spin over the span of the proton, reducing its effects.
- And finally, there are quantum effects that delocalize the quarks, preventing them from being in exactly one place like particles and requiring a more wave-like analysis. These effects can also reduce or alter the proton's overall spin.