What does Atticus say about mobs?

Thank you for using the Jiskha Homework Help Forum. In Chapter 15, according to Atticus, incident at the jail was a
demonstration of mob psychology. People in a group, he says, will do
things that are unworthy of them as individuals. By singling out Mr.
Cunningham, Scout made him start acting as an individual again, not
just another face in the mob.

So what exactly is psychology because all I get is how it's used not the real term!