It K/F is a Kummer extension then there is a nice way to think of K i.e. K is an adjunction of n-th roots of F. But in order to be a Kummer extension we need F to have enough roots of unity, in fact, it must contain all roots of unity of n. The idea of class field theory is to remove or at least ease the restriction that F has to contain all the roots of unity and try to generalize the notions of Kummer extensions.