Think of a reversible reaction in a container that is sealed, so nothing can escape. At the start there are lots of reactant particles, so the forward reaction is fast. There are no product particles yet, so there is nothing to react in reverse. As reactants are used up the forward reaction slows down. As products build up the reverse reaction speeds up.
Eventually the two rates become equal. Apparatus that prevents the escape of reactants and products is called a closed system. In a closed system, equilibrium is reached when the forward and reverse reactions occur at exactly the same rate. In an open container a gas could escape, so equilibrium would not be reached.
At equilibrium both reactions are still happening, so it is called a dynamic equilibrium. Reactants are still turning into products and products are still turning back into reactants, but at the same rate. This means the amounts of reactants and products stay constant. It does not mean they are equal, because there may be far more reactants than products, or the other way round.
