The actuator is the main workhorse of any doorbell. It has two electromagnetically controlled plungers that hit the chimes to produce the typical "ding-dong" chime so familiar to us all. One chime is the "ding" while the other is the.......uh......"dong". (why did I just think of Beavis and Butthead while typing this?)
The magnets should stick to the metal chimes and you want them to be placed so the plungers strike the magnets when you press the doorbell.
This makes the magnet deaden the sound of the chime to the point that you hear the ding-dong but the really is no sustain to the notes. It's more of a "did-dud" and it's very quick.