The LNB is the component located at the end of the arm projecting from the satellite dish. It converts the Ku-band signal beamed from the satellite to a 3.7 - 4.2GHz signal, then filters out low-end frequencies and amplifies the high-frequency signal before sending it to the LNB's coaxial output(s). Satellite service providers (such as DIRECTV) have satellites in multiple orbital positions, and a separate LNB is needed to access each satellite position. Essentially it just 'tunes' the feed from the satellite.