What does this component do?
The device FSQ0270 is an integrated controller for Switched Mode Power Supplies. The datasheet provides standard application circuit schematics. 

When used in such a circuit, the device is fed with rectified mains voltage (unregulated DC half-sine wave), and generates the pulses required to drive a suitable transformer at 100 KHz, to provide a desired DC output voltage. 

The Feedback pin accepts input from an optocoupler to sense the output current, and manipulates the output cycle-by-cycle (at each pulse, at 100 KHz i.e. 100,000 times a second) suitably to provide the designed voltage, with current limiting.

The part also has built-in safety features such as overload protection, overheat protection, and over- and under-voltage protection. 

Some context on why the question was raised, would help provide a more relevant response.