What does fullness in the left adrenal gland mean?

Different things: Usually means enlargement of adrenal gland. Enlarged adrenal gland can be seen in benign and malignant tumors, overgrowth or hyperplasia, infection, cyst,hemorrhage etc. Sometimes CT and blood tests are used to assess adrenal configuration and function.

Large: The gland is enlarged. Usually benign caused by hyperplasia, usually as a reaction to some medicine or systemic process/illness. Could be a tumor. If so, usually benign, likely an incidental adenoma of no consequence. But needs to be evaluated further.