How do reactants and product concentrations in a system at....?

In a dynamic equilibrium, both the forward and reverse reaction are constantly occurring. They are at equilibrium because the rate of the forward and reverse reactions are the same. This means that the concentrations of both reactants and products remain the same.

The equilibrium can be disturbed by one or more of the following
1. Changing the temperature
2. Changing the concentration of one of the reactants or products.
3. Changing the pressure (if there are gasses in either the reactants or products.

Some will argue that adding a catalyst will change the equilibrium, but they are wrong, all this will do is to reduce the time taken to reach equilibrium.

This is down to the Le Chatalier's Principle
Whatever is done to a system in equilibrium, the equilibrium moves to "undo" the imposed change.