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 a SCART or composite input, so that compatible devices can bypass this problem.