Zeolite is a porous absorbent mineral which is used in aquarium to absorb ammonia from the water.
Ammonia is released from fish waste and uneaten food in an aquarium. As part of the nitrogen cycle it is broken down in to nitrites and nitrates. However, if the nitrifying bacteria are not properly established or if more ammonia is released than usual due to excess fish in the aquarium, a dangerous ammonia spike can occur. This can kill fish.
Hence to absorb the ammonia on a short-term basis, Zeolite is used. Zeolite acts as a sponge and absorbs the excess ammonia. After the Zeolite is exhausted it has to be replaced.
While there are many different types of zeolites available, you must choose one which is specifically meant for aquarium use. Further, there are different zeolite types for freshwater and saltwater aquariums.