As it pertains to valence electrons, "8 is great" means the valence electrons present in the orbital confers stability on the atom as the shell becomes completely filled.
For an atom to be stable, it must have completely filled outer orbitals.
The number of electrons required is usually 8.
This is the octet number which confers special stability on an atomic configuration.
Elements having this number of electrons in their outer shell are unreactive.
