In 1955 Jean-Pierre Serre, motivated from Algebraic Geometry, stated his famous conjecture, “We don’t know if there exist finitely generated projective modules over the ring of polynomials with coefficients in a field which are not free”. Soon this conjecture became one of the most challenging problems in Abstract Algebra. In 1976 Quillen and Suslin managed to give two independent solutions of the problem and the theorem saying that there are not such projective modules carries their names. In this talk we will focus on understanding what every single word of Quillen-Suslin theorem means and sketching a proof given by Varestein.