It is usually made of foam rubber, rubber or accordion paper. This keeps the top of the cone aligned. The weight of the paper and the spring of the suspension make up the 'natural resonance' of the speaker. In other words if you tapped the cone it would vibrate at the same speed no matter how hard it is tapped. That 'natural resonance' is changed once a signal is applied and the cone starts to move, i'll explain this more later.
Next glue is applied to the frame and the cone is dropped into the frame.
Now you can see the plug flush with the bottom of the cone. Now we glue the dust cover on, the part that always seems to get punch in.