Without a predator the population of elk goes unchecked.
Overpopulation causes an imbalance in the ecosystem and the elks food (plants) all get eaten and do not get a chance to grow.
With the return of the wolves however, the elk population is reduced.
Balanced Ecosystem = Sustainable Ecosystem
Now that there are not as many deer some of the plants get to grow to a size out of the elks reach.
The plants are now big enough to grow and spread there seeds, which provides new food for the elk
These new plants are potential food for the newborn elk, and these newborn elk become potential food for the wolves... This is a balanced ecosystem because it is sustainable for all organisms involved.