Immediately after the Big Bang, the Universe was extremely hot and dense. Under these conditions, atoms could not exist, and the matter was distributed as a highly ionised plasma. As the Universe expanded, however, its density and temperature dropped until (after about 380,000 years) the conditions were such that ions and electrons could ‘recombine’ to form atoms (mostly hydrogen and helium). This is known as the ‘epoch of recombination’.
We cannot actually observe the Universe before the epoch of recombination, since the ionised plasma that existed before this time was very efficient at scattering radiation. The result is that information about the early Universe (information astronomers detect in the form of photons) was not able to escape, hiding the early Universe from current observational strategies.
At the epoch of recombination, however, the ionised plasma gave way to neutral atoms, which did not scatter the photons but allowed them to travel freely. All photons emitted since this time have also been able to travel unimpeded, allowing astronomers to study objects all the way back to recombination. The oldest photons detected – those that were emitted at the epoch of recombination – make up the cosmic microwave radiation.