The mold that grows in your mold terrarium feeds on the bread, cheese, and other foods. The mold feeds itself by producing chemicals that make the food break down and start to rot. As the bread rots, the mold grows. It can be annoying to find moldy food in your refrigerator. But in nature, mold is a very useful thing.
Mold helps food rot, which is an icky but necessary thing.
In a natural environment, rotting things return to the soil, providing nutrients for other plants. Mold is a natural recycler. There are thousands of different kinds of molds. One mold that grows on lemons looks like a blue-green powder. A mold that grows on strawberries is a grayish-white fuzz.