According to Big Bang theory, temperatures and pressures for the first ~380,000 years of the Universe were such that atoms could not exist. Matter was instead distributed as a highly ionised plasma which was very efficient at scattering radiation. The result was that information (photons) from the early Universe were effectively trapped in an impenetrable 'fog' which, to this day, hides these early times from astronomers.
The term primordial fireball refers to this early time in the Universe. As the Universe continued to expand, its temperature and density dropped, allowing for the formation of atoms. This is known as the 'epoch of recombination', and it was at this time that photons could travel freely throughout the Universe for the first time. The Cosmic Microwave Background radiation (CMB) is the record of these photons at the moment of their escape.