CMB - or ‘Cosmic Microwave Background’ - is the thermal radiation filling the observable Universe - the echo of the start of the Universe. Currently, we can see 380,000 years after ‘The Big Bang’; that’s around 13.37 billion years ago!
It is the radiation left over from an early stage in the development of the Universe; when the Universe was young, before even stars and planets, it was a lot smaller and a lot hotter, filled with a glow and a fog of hydrogen plasma. As it expanded, both grew cool enough that protons and electrons could form neutral atoms, and so did not absorb the radiation. This meant the Universe became transparent instead of being an opaque fog.
The glow is nearly uniform in all directions, but the tiny variations show a pattern which is equal to what would be expected of a hot gas that has expanded to the current size of the universe. From the data taken, a map of the Universe is formed; the WMAP image.