As reactants react products are made, which increases the concentration of the products. As we know, the higher the concentration the faster the rate of reaction. The forward reaction (the reaction of the reactants) slows down while the backwards reaction (reaction of the products) speeds up because the concentration of products is increasing. Eventually, the rate of reaction, both forwards and backwards will be the same. The system is at dynamic equilibrium now, as the concentrations of both reactants and products remains the same. An example of a reaction that has dynamic equilibrium is the Haber process.