What is Degenerate levels?

Example 1.15 (Degeneracy and symmetry)

The level $n_x^2 + n_y^2 + n_z^2 = 6$ of the cube comes from $(2,1,1)$, $(1,2,1)$ and $(1,1,2)$: $g = 3$. Stretch the box slightly along $z$ and the third function moves away from the other two: the degeneracy came from the symmetry of the cube. The same link between symmetry and degeneracy organises Chapters 4 and 5.