[Mitsuru only knows the basics, but based on the sense of confidence in his words, it would be easy to assume he is, somehow, an expert in the subject.]

It's the balance of organic life, like plants and animals. If it goes out of balance, it could cause areas to become uninhabitable. For example, without enough plants to take in carbon dioxide and convert it to oxygen, there wouldn't be enough oxygen in the air for people to breathe.