The LNB is usually affixed either in or on the satellite dish.  The purpose of the LNB is to utilize the super heterodyne effect; and amplify and convert a wide block (band) of frequencies.

This helps compensate the signal loss associated with typical coaxial cable at relatively high frequencies.  The term 'low noise' relates to the quality of the 1st stage input amplifier transistor, measured in either called Noise Temperature units, Noise Figure units or Noise Factor units.