What is meant by equilibrium constant?

Equilibrium constant is generally denoted as K, which is the ratio of concentration of product and reactant, when a reaction reaches to an equilibrium state.

For a general reaction 

A + B → C + D

Here, K = [C][D]/ [A][B]

Physical significance of equilibrium constant is as follow:

1) The value of equilibrium constant gives information of the extent of the reaction.A large value of equilibrium constant indicates the greater extent of forward reaction.

2) If the value of equilibrium constant and initial concentration of reactants is known, we can determine the concentrations of various species at equilibrium.

3) The degree of dissociation of a substance can be calculated from the value of equilibrium constant.