Microbes are microscopic organisms, including bacteria, viruses and small multicellular organisms. On Earth microbes live almost everywhere: in soil, in water, inside animals and humans. On Mars there are no known microbes.
Possible Martian microbes
The Martian surface is utterly sterile due to radiation and the absence of liquid water. The environmental conditions seem not to allow them to survive. Subsurface environments, however, may be favorable to microscopic life.
There are speculations about a threat towards the settlers or even towards Earth in case of return missions. And there are strong arguments against a threat. Microbes are always adapted closely to their natural host. In rare cases a microbe manages to jump from one terrestrial species to another. But this is rare. And this applies only for microbes that are already accustomed to live in hosts. The terrestrial soil microbes never jump to an animal, because they have adapted to the soil environment, and their whole metabolism depends upon this environment. With Martian microbes a jump is even more unlikely. See Back contamination for further discussion.
Furthermore, some people want to protect a possible indigenous Martian sub-surface biosphere from contamination by terrestrial microbes, which can only be avoided with an effort.