bangs are commands you can send to modules. e.g.:

this would move a certain module to position x y.
every setting a module lets you change at runtime can be accessed via !bang, some modules have more, some have less, and you simply use them to make themes more dynamic. for example, often a theme needs to have a dynamically resized taskbar, to guarantee the systray next to it fits in, so if an icon is added into the tray, a bang is called to make the taskbar smaller.

i hope it was clear enough... :)