Satellites use comparatively high radio frequencies to transmit their signals. As satellite signals do not easily pass through walls, roofs or windows, satellite antennae are required to be outdoors, and the signal needs to be passed indoors via cables. When radio signals are sent through 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 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 block (or band) of relatively high frequencies 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. It is also much easier and cheaper to design electronic circuits to operate at these lower frequencies (rather than the very high frequencies of satellite transmission).
The "low-noise" part means that special electronic engineering techniques are used so that the signal has less noise (unwanted signals) on the output than would be possible with less stringent engineering. Generally speaking, the higher the frequencies with which an electronic component has to operate, the more critical it is that noise be controlled. If low noise engineering techniques were not used, the sound and picture of satellite TV would be very low quality, if it could even be received at all.
Direct broadcast satellite dishes use an LNBF ("LNB feedhorn"), which integrates the antenna's feedhorn with the LNB.