RF stands for radio frequency, and defines a band of electromagnetic radiation, popularly known as light, that is on a continuum of frequencies that above RF pass through visible light and on towards gamma rays.
In simple terms, whenever something possessing charge moves it generates an electromagnetic wave associated with the motion and at right angles to it. Visible light is generated by motions of electrons at an atomic scale, radio frequency is motion of electrons at the scale of antennas, but they both generate electromagnetic waves quantized as photons.
edit - darkmighty. Electromagnetic radiation at radio frequencies is popularly known as light in contexts such as speed, but not in contexts such as colour.
RF refers to specific frequencies of radiation, which are not popularly known as light at all. Radio is usually in the MHz range to a few GHz (10^6-10^9 Hz). Light starts a hundreds of Terahertz (10^14 Hz). The behavior is qualitatively different at usual scales due to quantum mechanical behavior (light is difficult to emit/absorb coherently -- it's more particle-like) at high frequencies.
A photon striking a semiconductor will probably displace an electron if given an adequate bandgap, creating a voltage spike (which is nonlinear -- i.e. the problem isn't a propagating terahertz wave obviously). An incident radio wave will cause voltage spikes through coherent induction in the wires (i.e. they act as an usual antenna).
Even if there is no fundamental reason to believe that quantum theory would make a distinction between "microwave photons" and "optical photons", this demonstration puts this equivalence across a huge frequency range on a firm experimental footing. Moreover, the lower frequency of the microwave photons enabled a more complete characterization of the effect than has been able so far with optical photons, opening up new possibilities to characterize radiation sources. Finally, the new experiment highlights how quantum optical effects can be exploited in experiments with microwave sources, which may lead to practical applications of "microwave optics".
