An LNB (Low Noise Blocker) is used in broadcast satellite reception. The LNB is usually fixed on or in the satellite dish.
Satellites use comparatively high radio frequencies to transmit their signals. As microwave satellite signals do not easily pass through walls, roofs, or even glass windows, satellite antennas are required to be outdoors, and the signal needs to be passed indoors via cables. When radio signals are sent through coaxial cables, the higher the frequency, the more losses occur in the cable per unit of length. The signals used for satellite are of such high frequency (in the multiple gigahertz range) that special (costly) cable types or waveguide would be required and any significant length of cable leaves very little signal left on the receiving end.
The job of the LNB is to use the superheterodyne principle to take a wide block (or band) of relatively high frequencies, amplify and convert them to similar signals carried at a much lower frequency (called intermediate frequency or IF). These lower frequencies travel through cables with much less attenuation of the signal, so there is much more signal left on the satellite receiver end of the cable.
For further information see http://en.wikipedia.org/wiki/Low-noise_block_converter