Cooked sweetcorn (in game name "Sweetcorn") is a food item, made by cooking sweetcorn. Cooking it gives 104 Cooking experience. When eaten it heals 10% of your total hitpoints, rounded up.
Cooked sweetcorn may also be put in a bowl (giving Sweetcorn), and then mixed with a cooked tuna, with a knife in your inventory at level 67 Cooking, to make tuna and corn. This mixture heals 13 hitpoints when eaten, but may also may be added to a baked potato with butter at level 68 Cooking. This makes a tuna potato, which heals 22 hitpoints.