They are small crustaceans that reproduce in aquariums. Just basically think of them as small bugs living in your tank that are a sign of a healthy tank
Pods, short for copepods or amphipods, are basically the "bugs" of the underwater world. The ecosystem relies on them, and many small fish, invertebrates, and corals eat them.
You can culture them separately, or put them in your tank and let them reproduce, which they will do readily.