A chemical equation is said to be at chemical equilibrium when the rate of the forward reaction is equal to the rate of the reverse reaction. The forward reaction is that of the reactants combining to give the products and the reverse reaction is that of the products combining to give the reactants.
Let the rate of the forward reaction be represented by Kf and the rate of the reverse reaction by kr.
For a reaction A + B <---> C + D, at equilibrium kf[A][B] = kr[C][D]
The equilibrium constant Kc = kf/kr = [C][D]/[A][B]. This is a constant value for each chemical reaction.
Kc is the same irrespective of the initial concentrations of the reactants. No matter what the initial concentration of the reactants is, the concentrations at equilibrium are such that the ratio of the product of the concentration of the products and the product of the concentration of the reactants is equal to Kc.