(Radio Frequency in) Refers to coaxial sockets on the back of TVs, VCRs and DVDs that accept analog video signals. The "RF in" socket is typically connected to an antenna or line from the cable company that is transmitting UHF/VHF TV signals. Contrast with RF out. See F connector and RF.
The "RF in" socket on this VCR is picking up UHF/VHF TV signals from an antenna or cable. The "RF out" is the output of a VHS tape.