According to PC Magazine, the RF input is the standard input used to connect a digital television antenna to a television using a coaxial cable. RF stands for radio frequency connector. RF connectors are designed to work at radio frequencies in the multi-megahertz range.
Mechanically, the RF input provides a fastening mechanism. B&H Photo and Video explains that the main reason to use the RF connector is to take advantage of a television’s internal tuner rather than use a separate tuner box. However, as of 2014, the vast majority of television viewers receive channels exclusively through a cable or satellite subscription, and the tuner is built into a separate component like a digital video recorder rather than the RF input of a television.