A biomass pyramid is a graphic illustration of the mass of organisms at each trophic level, which tends to get less as it goes up from producers to each level of consumer. There are inversions, however, where the mass of consumers is greater than the mass of producers.
Biomass pyramids tend to reflect energy pyramids, since an energy pyramid reflects the amount of energy stored in organisms' bodies at ascending trophic levels. Biomass pyramids are predictable in cases such as most grasslands. In these, a very large mass of plants supports a smaller mass of primary consumers. These, in turn, support an even smaller mass of predators. The top predators in an environment also tend to have the least total mass of any trophic level.
In some marine environments, the situation is reversed. In these, the mass of microscopic algae is actually less than the mass of zooplankton, their primary consumers. This situation occurs because the zooplankton consume the algae very quickly. The situation is only stable because the algae reproduce extremely rapidly, allowing them to keep stable numbers and support the higher trophic levels despite an ongoing lower level of biomass. In these cases, the energy and biomass pyramids are different, since there is still more energy in the producer level despite the lower biomass.