Basic torque script language questions

Hi everyone, I would like to ask what most likely will sound like silly questions, but the only really dumb question is the one not asked.

I am begining the process of understanding torque script and would appreciate it if someone would explain in simple terms what a "callback" is. As an example...within MainMenuGui.cs, above function onSinglePlayer() the comment states "Callback from the shell button for triggering single player."

I am self taught in several languages and have an idea about it but would lke to get the info from you who know.