It is a nonlinear effect that makes a metamaterial transparent over a very narrow band of frequencies. This occurs in the vicinity of the metamaterial's resonance where it is typically absorbing. In addition to being transparent, crazy things happen in this transmission band like strong dispersion, slow waves, etc.
Actually, in the metamaterial theory Electromagneticlally Induced Transparency is not referred to a nonlinear effect. Indeed it is referred to a particular resonant state known as a trapped (or dark) mode resonance. At this resonance the field inside the system appears to be weekly coupled to free space leading to low transmission losses and thus the resonance has exceptionally high quality factor. For the first reading I can recommend these papers