Topping the plant means to remove its primary growing shoot. The plant then redirects its energy to the next two closest shoots which become primary shoots. This may be done as many times as the grower desires. Note, however, that the resulting buds will be smaller than if the plant had been left untopped. The real issue becomes "Does the mass of the two new buds exceed the mass of the otherwise untopped cola?" The answer to this question varies from strain to strain, perhaps even, from plant to plant.
When topped, frequently the two central growth tips will stop growing for a period, while the plant directs its energies to repairing the damage central shoot. A collateral effect of topping is that the plant bushes out, that is, all of the lower branches continue to grow when the growth of the central shoots pauses. This can be useful in a propagation technique like SCroG, in which it is desirable for all of the budsites to be on an even plane.
The slowdown in growth may be avoided by pinching,rather than cutting, the growth tip. This does not cause the resulting bushiness in the lower growth, but does cause the two secondary tips to become primary.
How, when and where to top are also matters of ongoing debate. Ultimately, it becomes a personal decision of the grower, based upon his own experiences and goals.