What does this effective nuclear charge equation mean?

What is the nuclear effective charge of a sodium (Na) atom?

Sodium has an electron configuration of [Ne]3s1. The nuclear charge is Z = 11+, and there are 10 core electrons (1s22s22sp6). We therefore expect S to equal 10 and the 3s electron to experience an effective nuclear charge of Zeff = 11 - 10 = +1. As an atom, protons are equal to electrons. So, the atomic number is equal to the number of electrons, which is 11 in this case. Because core electrons are most effective at screening a valence electron from the nucleus, the value of S is usually close to the number of core electrons in an atom (it's a little more complicated than that, but let's just make it simple for ourselves for now). There are 10 core electrons that "screen" (shield) the valence electrons from the nucleus due to electron-electron repulsions (remember, opposites attract. like charges repel). So, Zeff = Z - S can be simplified to Zeff = Number of Electrons - Core Electrons. Does this make sense? It's a little more complicated than this, but this is pretty much the basics.