The feedhorn is the part of a satellite dish system which gathers the reflected signal from the dish and focuses it towards the LNB.
It is a type of horn antenna that is deployed to convey radio signals between the transceiver and the reflector antenna. Horn antennas basically effect a transition between waves propagating through a transmission line like a waveguide and the waves propagating in free space. The shapes could be various, based on the final purpose of regulating some of the fundamental properties of gain, radiation pattern and impedance.
The feedhorn also does the job of attenuating unwanted signals from sources like adjacent channels. This is done by selection of the polarity of the waves that are to be received. As an accessory located at the focal point of a satellite dish system or a paraboloid antenna, it gathers the reflected signal from the dish and focuses it towards the Low Noise Block (LNB), which is usually affixed in or on the dish.