And because looking back in space means also looking back in time, observing this primeval light is like seeing the Universe as it was almost at its origin. The Universe is filled with a very special light. Also, the CMB does not originate from one object in particular, but from the whole Universe. It's actually the first light that existed freely in the Universe. That is the reason why it can be detected today as coming from everywhere in the sky.
Microwaves are a specific kind of light.
Light can be thought of as a wave, a wave that can be 'tight', 'compressed' like a closed accordion, or 'stretched' like an open accordion. The shape of the wave, and its energy, are described in terms of quantities called frequency and wavelength. For microwaves, that wavelength corresponds to centimetres or millimetres. This illustration shows the different kinds of light, or so-called electromagnetic radiation. The energy of the light increases towards the left, meaning that the shorter the wavelength of the light is, the higher energy it has.