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The universe is filled with radiation at a temperature of 2.728K, whose spectrum peaks at about 300GHz. This radiation was first detected several decades ago and is known as the Cosmic Microwave Background (CMB).
If we observe the microwave sky we find that the temperature of the CMB is not exactly the same in all directions: it is anisotropic. There are small fluctuations in the temperature across the sky at the level of about 1 part in 100,000: the microwave background temperature anisotropies.
In the Hot Big Bang Model the universe was much hotter and denser in the past. It has been adiabatically cooling as it expands. At early times the universe was almost entirely ionized. Photons and baryons (protons and electrons) were tightly coupled by Compton scattering and electromagnetic interactions.
At a redshift of about 1000 the universe cooled enough to form Hydrogen. With the rapid drop in the free electron density the photon-matter interactions effectively ceased.
What we see in the Cosmic Microwave Background (CMB) is a snapshot of the universe at redshift 1000. The fluctuations in temperature across the sky are the precursors of the large-scale structure we see around us today. These small fluctuations grow through gravitational instability from 1 part in 100,000 at redshift 1000 to highly concentrated structures today.
After several decades of searching, CMB anisotropies are now being ``routinely'' detected and mapped over a range of angular scales and frequencies (see here for more details).