A biological filter, also called a biofilter, consists of a space filled with bio media on which nitrifying bacteria can attach. To keep these filters compact, most people use media with a large surface area per unit volume. This unit of measure is usually referred to as the specific surface area (SSA) of the biofilter media. The higher the SSA, the more bacteria can grow.
A biological filter is part of a complete filter system for, by example, a koi pond. After debris has been removed from the water through a drum filter, the water goes to the biological filter. As the name suggests, biological nitrification takes place here. You are probably asking yourself what this means. Fish excrete ammonia through their gills, which then dissolves in the water. High concentrations of ammonia can lead to loss of appetite or growth, and in the worst case, even death. So the ammonia will have to be removed from the water. For this, you can use bacteria. These bacteria convert the ammonia into nitrite and then into nitrate. Nitrate is only harmful to fish in very large amounts, but these amounts do normally not occur.