A quad-ridge flare horn (QRFH) is a type of horn antenna with specific characteristics aimed at its utility, i.e., four ridges along the sides within the horn from the narrow portion to the wide portion. A radio telescope feed consisting of such an antenna is a QRFH feed.
A horn is a waveguide, i.e., a path for radio waves with metallic walls that will reflect waves back into the path and the term horn implies flaring or a tapered shape. Horn antennas can be designed to handle a wide frequency range, and ridges, as above, can be designed to improve the characteristics further. A dish telescope's feed is the receiver that converts radio signal to electronic, including any horn or waveguide bringing the waves to it, carrying out this function on the waves that have been concentrated by the reflective dish antenna.