Electricity.
Is a kind of energy. It has two different kinds of charge: positive charge and negative charge.
+ And +: repel.
-And +: repel.
+and -: attract.
Before rubbing, the pen has equal numbers of positive and negative charges. The changes when the pen is electrically neutral.
When the pen is rubbed with other thing, negative charges travel from the other thing to the pen. The pen now has more negative charges than positive charges. It has become negatively charged, because the negative charges have left the other thing, it now has more positive charges than negative, it has become positively charged.
Atoms contain negatively charged particles: electrons.
Positively charged particles: protons.
Neutral particles: neutrons.