CMB anisotropies are variations in the spectrum of the cosmic microwave background (CMB) across the sky, essentially variations in the temperature associated with its black-body spectrum. The variations are tiny, the CMB being about as perfect a black-body spectrum as is ever seen, but they are clues, both to recombination (when the CMB was released) and what led up to it, and the effects on the CMB of passing through the universe since. Anisotropies can be classified as primary anisotropies, stemming from recombination and before, versus late time anisotropies (aka secondary anisotropies) from the effects of the universe that the photons have since passed through. Anisotropy contributors (beyond Earth atmosphere, and foreground microwave emissions from the Milky Way):
(Diffusion damping is another primary contributor, but decreases the primary anisotropies.) The common analysis consists of a multipole expansion of the power spectrum ("power" for square of the variations as compared to the mean temperature), which is displayed as a graph of a function of power against the "l" (the degrees of the spherical harmonic modes).