Improve
Improve (or imp for short) is an action done on an item to increase its quality. To improve the item, right-click it, click "Examine", read what item is needed to improve it, activate the necessary item, and right-click the first item again to attempt to improve it.
Notes
Some items require certain conditions to be improved. For example, smithing items need to be glowing hot to improve. Also, most items need to be at 0 damage (repaired) to be improved, except wood fences, wood gates, and stone walls.
Some items cannot be improved and will reflect that when the examine command gives no information for improvement: newbie starter kit tools, most building materials (dirt, planks, etc), yoyo, unrepairable items, etc.