I don't understand what valence electrons and core electrons are.

Question

Ex: Se = 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p4

What makes 4s2 and 4p4 valence electrons and the rest core

Answer 1

Valence electrons are those in the outside shell, usually, and sometimes the last two shells.
Na is 1s2 2s2 2p6 3s1 and the 3s1 is the valence electron. YOu know that to be true since Na has a valence of +1 (always).
Mg is 1s2 2s2 2p6 3s2 Mg has a valence of +2 (always) and that is because it can lose the last two electrons in the 4s orbital.