The light which constitutes the cosmic microwave background comes from the surface of last scattering. This is, on average, the surface at which primordial photons last interacted with matter in the universe, or in other words, the point at which photons started free streaming. Similarly, the surface of the cosmic neutrino background, if it could be observed, would mark when neutrinos decoupled and began to stream freely through the rest of the matter in the universe.