My ct scan says "mild thickening of left adrenal gland." what does this mean?

Incidental findings: By itself, nonspecific. It can mean there is hypertrophy (normal variant) or an adrenal mass which can be benign or malignant. These incidental findings can be anxiety provoking but it's always better to know. Options include follow up CT or abdominal MR. Given findings are mild, I wouldn't be surprised if CT is repeated, likely with IV contrast. Discuss with your primary doc and radiologist.