Lateral right maxillary sinus 8 mm ap x 6 mm transverse x 9 mm craniocaudad mucous retention cyst versus polyp. What does that mean?

Depends. If it is asymptomatic and painless then nothing. A usual finding, especially in asthmatics. If there are symptoms, frequent sinusitis, or it is growing as noted on serial exams then removal is recommended with histological examination (biopsy).
Cyst in sinus. It means that you have a small soft tissue mass in your right cheek sinus. It if nearly impossible to differentiate solid versus fluid-filled masses on a ct scan. Either way, no treatment is generally necessary for this condition.