It means, it's typically stable with 8 electrons when we're discussing the stability of atoms in their most common, low-energy states. This third shell consists of 3 types of orbitals: s, p and d orbitals. The s orbital can hold 2 electrons, the p orbitals can hold 6 electrons (3 p orbitals x 2 electrons each), and the d orbitals can hold 10 electrons (5 d orbitals x 2 eletrons each.) When filled to capacity, this amounts to 18 electrons.