What does very minimal fullness in the mastoid cells mean?

Confusing: Fullness in the mastoid is a symptom, and using the term mastoid cells is a radiological diagnosis. I am guessing that your doctor ordered mastoid x-rays and the radiologist identified minimal degree of fluid in the mastoid air cells. The mastoid bone behind your ear only gets involved if you have a middle ear infection (hearing loss). So we need more specific information on your complaint.