Chemical equilibrium is the state in a reversible reaction where the forward reaction and the backward reaction happen at the same rate. At this point, the amounts of reactants and products do not change with time, even though both reactions are still happening.
It is a dynamic balance — meaning the system is active, but looks unchanged from the outside.
For example, when a gas turns into a liquid and the liquid turns back into a gas at the same rate, the system is in equilibrium.