Microbes are everywhere. When you culture microbes, you need to be very careful not to let any contaminating microbes from the environment land in your samples. If they do, they will rapidly grow and multiply in your nutritious culture media. You also need to be careful not to let any microbes from your cultures escape from your work area or be accidentally ingested.
If you follow proper sterile technique, you will grow only the microbes you intend to grow, and you will keep them in their proper place. Mostly, you want to avoid dust and fingers, the most common carriers of contaminating bacteria and molds, from entering your cultures.