Those are the labels of irreducible representations corresponding to the wave-function on that symmetry point, derived via group theory. They classify how wave-function at that symmetry point evolves under crystal symmetry operations, similar as the classification of odd and even function $$f(x)$$ by whether it changes sign under inversion operation of $$x$$. Here is a pretty nice introduction to this topic of group theory and band structure (if you have access to it):