An RF modulator (for radio frequency modulator) is a small device that takes an input signal and outputs radio frequency-modulated signals.
Most commonly these are found in home computers or video game systems that display graphics on a television receiver. The RF Modulator takes the internal RGB, YUV, or composite video signal from a video-circuit inside the machine and generates a PAL or NTSC broadcast signal that can be fed into the television aerial socket.
These give a relatively poor picture; some of the image quality is lost during the modulation in the source device, and some is lost again when demodulated in the television. Some televisions provide SCART-RGB or composite input, so that compatible devices can dispense with the RF modulator circuit and bypass this problem. However the aerial connector is the only one available on all TV sets, even very old ones.