Well, at equilibrium the net change of reactants and products is zero. Since the forward rate of reaction gives the rate at which reactants are transformed to products and the backward rate gives the rate at which products are transformed to reactants, if these two rates are not equal then the net rate of change of products and reactants will be nonzero.

As an example, consider a situation where I hand you 2 boxes per minute. If you hand me 2 boxes per minute, then the number of boxes we have stays the same and our system is in equilibrium. However, if you hand me 3 boxes per minute, boxes will accumulate on my side. Since the number of boxes on my side increases with time, this system is not at equilibrium.