Atticus explains to Scout that a mob is always made up of people. He is trying to explain that, although mobs appear to be a large group, perhaps even an intimidating group, they are made up of individuals who can make individual choices to change their actions. This is a message that Atticus has been sending to Scout throughout the book at various points. Every individual has the right to make his or her own choices.
In this particular mob, Mr. Cunningham made an individual choice not to continue to threaten Atticus. His choice influenced the choices of the other men in the group. This is a powerful moment because Mr. Cunningham made this choice based on Scout's innocent actions to begin conversing with him about his son, Walter. As a result, his opinion changed, and he then changed the opinions of the other men. It was a mob no more. Any small action can change the tide of a mob, either positively or negatively.