A measure of the quantity and size of fish in an aquarium compared to the theoretical ideal for an aquarium of that size with that filer. The normal idea is 1 inch of fish (in length) for 1 gallon of water. In the wild, the ratio is actually much smaller, amounting to one inch for several hundred gallons of water.
Exceeding this ideal puts extra stress on the filter, thus requiring more maintenance to ensure that the aquarium environment does not degrade and cause stress in the fish. Stress eventually kills humans. It kills fish in minutes.