Understanding DC-DC Buck converters
But a Low Pass filter is the one that attenuates frequencies higher
  than a desired frequency and passes the lower frequencies. So How does
  this definition fit in this case?


The important thing you haven't grasped is that the on-off square wave has an average level that is totally dependent on the duty cycle. This mathematically average level gets converted to a real dc level by the L and C low pass filter action.

Here's a square wave that rises to 3V from 0V (gold trace): -



The blue trace has a little bit of low pass filtering applied - can you see that if a lot more low pass filtering were applied, the blue trace would be almost a constant DC level of 1.5 volts?