Every – okay, almost every – object in the universe emits light. When astronomers talk about light, however, they’re not only talking about the small portion of the entire electromagnetic spectrum that humans can see, i.e., the optical range, but instead they’re talking about all of it: radio waves, infrared, visible light, UV light, X-rays, gamma radiation (listed here in order of decreasing wavelength). Physicists would call this “electromagnetic radiation,” but “light” also fits very well, because, in the end, the same laws always apply for reception. Resolution, focal length, etc., it doesn’t matter if you are using a giant dish or a telescope, the terms always have the same meaning.
A radio telescope, as should be obvious, detects radio “light” from celestial objects. This is interesting, because radio waves are barely affected by dust and other obstacles, which is different, for example, from optical light (which cannot pass through dust) or infrared (which cannot pass through the Earth’s atmosphere). The ideal, of course, is to observe an unknown object in all wavelengths. All-wavelength observations is actually the big trend right now in astronomy; the technical term for it is “Multi-Messenger Astronomy.”