We have a simple reaction that is at equilibrium.
A <-----> B
In chemistry, the dynamic equilibrium between a forward reaction (from left to right or A ---> B) and the reverse reaction (from right to left or B ---> A) is a chemical equilibrium where the rate of the two reactions are equal. That means that the forward and the reverse reaction are happening at the same "speed".
When we reach the equilibrium the ratio between the concentration of products and reactants remains constant over time.
A -----> B
We are at equilibrium and we have 5 molecules of A and 5 molecules of B. In the dynamic equilibrium that we were working, at the same time that two molecules of A are reacting to become B, two molecules of B are reacting to become A and mantain the equilibrium.