What is cosmic microwave background radiation?

The CMB, taken by the Planck satellite. By European Space Agency, CC BY-SA 4.0.

In 1965 Arno Penzias and Robert Wilson discovered a faint, almost uniform, background glow that seemed to be present across the universe, without an apparent source.

What they had accidentally discovered was radiation left over from the beginning of the universe. However, this description is not entirely accurate.