As reaction proceeds concentration of reactants decrease and concentration of products increase, thus rate of forward reaction decreases and rate of backward reaction increases with time and after some time rate of forward reaction equals to rate of backward reaction.
Rateforward = Ratebackward
That means that amount of products formed by forward reaction is equal to amount of products consumed in backward reaction, similarly amount of reactants consumed in forward reaction is equal to amount of it formed by backward reaction, thus net concentration of reaction species remain constant and rate of two reactions also get constant. Thus is termed as state of chemical equilibrium.
