Microbes are microscopic organisms. On Earth microbes live almost everywhere: in soil, in water, even inside animals. On Mars there are no known microbes. The environmental conditions seem not to allow them to survive on the Martian surface.
Future settlers will bring microbes from Earth to Mars. They will live inside greenhouses, inside humans and animals. Microbes are essential for many atmospheric processes, including the Nitrogen Cycle.
Microbes can be used in biotechnological factories to produce chemical substances, for instance, proteins.