What is the cosmic microwave background?

Answer

It was released about 380,000 years after the Big Bang, when the universe cooled enough for light to travel freely.

Context

For roughly the first 380,000 years the universe was a hot, dense plasma of free electrons, protons and light, and photons scattered constantly off the free electrons, so the cosmos was opaque. As expansion cooled it to about 3,000 kelvin, electrons joined nuclei to form neutral atoms, an event called recombination. Light could then travel freely, and those photons have been streaming through space ever since.

Expansion has stretched that light enormously. Its wavelength has grown by a factor of about 1,100, shifting it from visible and infrared light into microwaves; today it corresponds to a blackbody temperature of about 2.725 kelvin. It reaches us from every direction and is nearly uniform, but tiny variations of about one part in 100,000 mark slightly denser regions that later grew into galaxies and clusters.

Arno Penzias and Robert Wilson found the signal in 1965 at Bell Labs while tracking down unexplained noise in a radio antenna, and later received a Nobel Prize for it. Satellites COBE, WMAP and Planck then measured its spectrum and ripples with increasing precision; COBE showed an almost perfect blackbody curve.

A misconception is that the background comes from a location far away. It comes from the whole early universe, so the 'surface' we see is a sphere around us at the distance light has travelled since recombination.