Humus is formed by decomposers such as bacteria, fungi, beetle, ants etc which falls under the saprotrophs. These decomposers decomposes or degrades the dead matter fallen on the forest floor such as leaves, plant parts, fruits, dead animal remails, fecal matter etc and converts it into dark colored substance. This dark color substance is called Humus.
Humus forms the top layer of the soil. It is very rich in nutrients. Eventually it gets absorbed by roots of plants from soil and therefore, it increases the soil fertility.
Remember: Based on how animals, plants and organisms obtain their food, they are divided into different categories such as autotrophs, heterotrophs and saprotrophs.
Saprotrophs : (sapro = dead); feeds on dead or decaying matter; e.g: yeast, bacteria, fungi, mushroom, beetles, ants, insects
This video explains what is humus from the chapter 17 of NCERT science class VIII in Gujarati for English medium.
Forests: Our Lifeline | Part 1 | Forests, Shrubs, Herbs, Creepers & Climbers
NCERT Science Class VII | Chapter 17 | Forests: Our Lifeline (Gujarati Explanation for Eng Medium) 2
You can find this chapter online on NCERT e-book here : NCERT Science Class VII Chapter 17.