Having a "colonized" bacteria means it has set up house, so to speak. It has found the proper environment to grow, reproduce and increase in number. An example would be the "colonization" of the nasal passages with MRSA. This bacteria can then be transferred to other parts of the body, to other people and to fomites (inanimate objects which can harbor the bacteria, like a fork or a doorknob) leading to the possibility of infection.

Colonization specifically refers to harboring the bacteria, without necessarily having signs of infection. Our bodies are covered by millions of bacteria all the time. Pathological bacteria can reside on our bodies at times without causing disease.