The precise measurements of the Cosmic Microwave Background (CMB) by the Planck satellite have pushed the understanding of the universe to unprecedented limits. The aim of the talk is to introduce the concepts, assumptions, limitations and prospectives behind this success. Some notions of General Relativity, the Cosmological Principle, an outline of the contents of the universe, the different epochs the universe has undergone, the basics of cosmological perturbation theory and how all these matters affect the measurements of the CMB will be presented. Finally, I will show how the CMB can give light on fundamental questions as what is the dark matter nature or what is the precise mechanism that generated the primordial perturbations that seeded the structures we observe today in our universe.