The magnitude of an equilibrium constant is a measure of the yield of a reaction when it reaches equilibrium. A very large value for Kc (Kc >> 1) indicates that product concentrations are much larger than reactant concentrations when equilibrium has been achieved: nearly all reactants have been converted into products. If Kc is large enough, the reaction has gone essentially to completion when it reaches equilibrium.
Earlier we defined a product-favored reaction as one that proceeds spontaneously in the forward direction when all concentrations (or partial pressures) have the standard-state value of 1 M (or 1 bar). If Kc > 1, then the concentrations of products are greater than the concentrations of reactants at equilibrium. Therefore, when all concentrations are at 1 M, the reaction would move towards producing greater concentrations of products in order to reach equilibrium. That is, the reaction proceeds in the forward direction. Thus, Kc > 1 (or Kp > 1) means that a reaction is product-favored at equilibrium.
A very small value of Kc, (Kc << 1) indicates that equilibrium is achieved when only a small fraction of the reactants are converted into products. Such a reaction is reactant-favored. If Kc is small enough, essentially no reaction has occurred when equilibrium is reached. By an argument similar to the one in the paragraph above, Kc < 1 (or Kp < 1) means that a reaction is reactant-favored at equilibrium.
When Kc ≈ 1, both reactant and product concentrations are significant and it is necessary to use the equilibrium constant to calculate equilibrium concentrations.