What happens when a reversible reaction is at equilibrium?

A reversible reaction is one where the reaction can occur both ways. Therefore, in a closed system where nothing can escape, some products made will always react in the reverse direction to make the reactants. At equilibrium, the rates of the two opposite reactions become the same, as a result the amounts of reactants and products in the mixture become constant. Now according to La Chatelier's law, if you try to disturb this equilibrium, the system will react in a way to oppose the changes you make. So, if you raise the temperature, the position of equilibrium will favour the reaction that uses up more heat to restore the temperature to the set point. The same with other conditions such as pressure, density, adding/removing reactants/products.