Roots becoming bound is a problem when too small a pot is used and the plant is not transplanted into a larger one. The growth of a plant that is root bound will be slowed and may cease entirely. Prolonged instances usually result in the slow decline of the plants health until death. No amount of feeding will correct a rootbound plant and transplant is the only viable course of action. Usually the first symptoms of a plant being root bound is slowed growth, particularily if this does not correspond with the plants normal rate of growth and feeding thresholds. Later signs include the ability to see roots emerging from the top of the soil and out the the drain holes at the base of the pot.
This really becomes a problem during the flowering cycle when the plants potential yeild and potency will suffer greatly if it is rootbound, yet the stress of transplant will as well. This is why I always transplant into a very large pot after sex is determined.