Scrapping is the action of destroying a unit by selecting it and hitting the delete key. A warning screen prevents accidental destructions. Multiple units can be scrapped all at once.
Scrapping usually returns 10% of the metal cost for the scrapped unit (less in some specific cases, like Ion Cannon). It's a way to relocate turrets, scouts or other cheap ships. It also the way to "open" some capturables like Zenith Reserve and Distribution Node.