The CMB Power Spectrum Cℓ encodes the statistics of temperature fluctuations on the last scattering surface (z ≈ 1100). The acoustic peaks arise from pressure waves (sound) in the photon-baryon fluid: modes that were at maximum compression/rarefaction at decoupling appear as peaks.
Peak positions constrain the geometry of the universe (flat → Ωtot=1; first peak at ℓ≈220). Peak heights reveal baryon/dark matter ratio — more baryons suppresses even peaks. The Silk damping tail at high ℓ shows photon diffusion. The measured spectrum (Planck 2018) is one of the most precise measurements in cosmology.