When we look out in the sky, we're actually looking backwards in time. Light from more distant objects take longer to reach us and thus we are observing now how they appeared in the past. We can see back a few billion years with the light of galaxies. The microwave light of the background shines from long ago in an infant universe 300,000 years old (the epoch of "last scattering") and illuminates the particle soup that existed before this time. This soup has a very smooth consistency and is composed of fundamental particles like electrons, protons, helium nuclei, neutrinos.
The obvious questions are: how did the universe go from a smooth particle soup to a complex system of galaxies and large scale structure. Can we use the fact that we're seeing the surface of this soup in the microwave background to help us understand this question.