How DFB(Distributed Feedback Laser )laser diode works?

today i was studying DFB laser,i know in it's active region there is Diffraction grating.
you can see image DFB

it is talked it has feed back,it is ambiguous !
can any explain how it can select a special wavelength ?
in this video <iframe class="restrain" title="YouTube video player" width="640" height="390" src="//www.youtube.com/embed/EuWI1WPrZ2I?wmode=opaque" allowfullscreen="true" frameborder="0"></iframe>
my question in under diffraction grating in active region why other wavelength can not pass?

in advance thanks for attention

in a laser, the light quantity increases with each pass thru the material. It has to do with electons turning into photons, etc.

SO, if you have a standard diode laser, you just cleave the rectangular shape of it so that the wavelength of the long side is approximately equal to the physical length. THEN you can have light of the correct color (wavelength) build up with each bounce off of the cleaved end. The only problem is that the exact wavelength of the light generated is not very stable. It can be one color, then suddenly hop to a slightly new color, or can even put out multiple colors (multiple lines) at the same time.

SO, if you want ONE very pure color of light (only one spectral line) instead of cleaving the edges to form a reflective surface, you make a printed bandpass filter that reflects one color of light very well. That way you get ONE spectral line width (mostly) instead of just a bunch of colors.