Humus (or humous) is the organic matter in the soil. It is made of dead parts of plants and animals. Humus takes in water and has plenty of nutrients. Nitrogen is the most important. Plants put roots into the soil to get the water and nutrients. Humus is good for plants because it is like a sponge.
The dark colour of humus (usually black or dark brown) helps to warm up cold soils in the spring.
How this happens change
When plants and insects die they are first raw organic matter. Bacteria and fungi eat the raw material, but they do not eat all of it. They leave a small amount behind. The part they leave behind is named "humus". This activity is dynamic and keeps happening. There is always more dead material arriving on top. So the soil always has organic material in it, along with small pieces of rock.