The equilibrium constant describes the relationship between the amounts of the reactants and the products at a certain temperature. You'll need to know the equilibrium constant as you study Chemistry II. For the general equilibrium:

the equilibrium constant expression is:

In the expression, K is the equilibrium constant, the subscript c indicates this constant is expressed in terms of concentrations (not pressures, p), the brackets (as usual) stand for molar (moles/L) concentration, the uppercase letters are the reactant and product species, and the lowercase superscripts are the coefficients in the balanced chemical equation.