A rainforest could be described as a layered symbiotic ecosystem that gets a tremendous amount of rain. The canopy is the top layer and composed of mainly epiphytes; climbing vines, orchids, bromilliads and such which serve as a protector, preventing erosion and trapping the moisture. The mid-story is mainly trees and shrubs that raise up the canopy, providing support for the canopy. The tropical floor of the tropical rainforest is primarily the domain of the decomposers!
The decomposers are last, but not least. They break down all the dead parts of the plants about them into essential nutrients that the living plants need. With out this happening the leaves and branches would accumulate and smother out the seedlings and other plants, and the bigger plants would starve for nutrients even when they were plentiful because the compounds were not in a form that could be absorbed.
So what are these decomposers found on the tropical floor of the rainforest? They are mostly saprophytes, also called agaric folia, or better known as fungi! There are over 400 species found in the Lesser Antilles ("Agaric Folia of the Lesser Antilles" by Pegler) alone. In Central and South American jungles they would number in the thousands. There are small, microscopic fungi hidden in leaves and under dead branches. Then go all the way up to puffball mushrooms, over 12 inches across, but those aren't the biggest, just the most visible. In some places on the planet there are subterranean mushrooms that weigh several tons and bigger than a pod of whales ( Check out this site of a really huge mushroom beneath the rainforest floor in Oregon)!
The floor of a tropical rainforest is also home to numerous other living things also eating the detritus of the canopy and mid-story. These include termites, beetles larvae, bacteria, and nematodes., they are all adding essential nutrients. Then there are also small plants such as lichens, club mosses and ferns, though some ferns have solid "trunks" that tower up over 60 feet reaching though the mid-story and into the canopy!
All these things result in the floor often seeming spongy in texture. It is also damp and lush, germinating spores and seedlings frequently within a few hours after they fall. Without the "floor", many of these plants would die. But that is what a floor is, a place or support where everything stands and most things begin, and that applies to the tropical floor of the tropical rainforest as much as anywhere.