Cosmic microwave background (CMB) radiation is weak microwave electromagnetic radiation detectable in all directions, presumed to be the result of the Big Bang. It is quite uniform (showing just 0.04% wavelength-variation in its peak strength, around the celestial sphere) but the variation that exists (anisotropy) is studied as a means of studying the early universe. It was discovered by accident in 1964 by Arno Penzias and Robert Wilson, earning them the 1978 Nobel Prize, having been predicted in 1948 by Ralph Alpher and Robert Herman.
The photons actually date to the time of recombination, representing a "picture" of that time, at about redshift 1090. At that time, the universe became transparent as the combination of electrons and protons throughout space yielded neutral hydrogen, which is much less likely to capture or scatter photons. The spectrum is basically a redshifted 3000K black-body spectrum, appearing now as a 2.725K black body spectrum. The very slight deviations from this are very much studied to answer questions about conditions during the formation and transmission of the radiation.
Other types of radiation (cosmic background radiation) have also been found. (The phrase cosmic background radiation is sometimes used to mean specifically the CMB.)
The phrase CMB foreground refers to microwave emission from nearer sources at the same frequencies. Such nearer sources are of interest to CMB researchers so they can be accounted for to work out an accurate picture of the CMB.
In planetary astronomy, the initials CMB are also used to abbreviate an entirely different phrase: core-mantle boundary, i.e., the border of a planet's core.