UV light is electromagnetic radiation with wavelength shorter than that of visible light. The frequencies are invisible to human eyes but visible to many insects.
Insects are capable of detecting UV and colors using photoreceptors. Bees, flies and ants are able to simultaneously receive information from the wavelength and e-vector (vector representing the electric field of an electromagnetic wave) of incoming light using its receptors.
The Glue Board Fly Trap attracts flying insects by using energy-efficient UV light. Once flying insects are lured, the glue board takes care of the rest.