What does "cluster of lymph nodes in the left posterior cervical region. Clinical correlation is recommended." means?

Not much. All it means is that you have some enlarged lymph nodes in the back of your neck. When they say clinical correlation it just means that they cannot comment on the lymph nodes other than to say they are large without knowing what the patient's history is. A 26-year-old with a large lymph nodes in the back of their neck usually has them because of an infection in the back of the nose.