This is an Arduino compatible AC voltage dimmer. Microcontrollers also can be used instead of Arduino. With this dimmer, you can control the speed of AC fan & brightness of light (only incandescent light). Check out the example code given in the documents section to understand its use.
This module cannot be used with energy saving bulbs, only incandescent bulbs. This module is very hard to control with Raspberry Pi, you need to know interrupt handling and timer ISR. It would be better if you could use this one: