What is the meaning of sphenoid sinus mucosal thickening?

The best laymen's term would probably be “stuffiness”. All of the sinuses (look in an illustrated anatomy book) are lined with mucous membranes just like what is in your nose. Thickening is usually due to allergies, irritants like smoke or dust, or infections from bacteria or viruses. It may be chronic (which means present for a long time) or acute (shorter time, but not necessarily more severe). Rarely, thickening can be due to cancer. Hope this helps.