That means that one (s) type orbital and three (p) type orbitals (all of the p orbitals in the valence set) have been mixed together to form four NEW atomic orbitals, all of equal energy, that point in specific directions (to the four vertices of a tetrahedron) the directions that the orbitals point is THE point of doing this because that enables you to explain why certain compounds like CH4 have a tetrahedral structure. It's kind of circular logic that you have to basically accept, but it makes sense if you delve deeper into the theory