A ship chunk is a 2x2 sized piece of destroyed ship. It cannot be hauled to a stockpile zone but only disassembled at its location for steel and components. They appear since the beginning of the game in the Crashlanded scenario, or fall from the sky in a mini event. You will be notified about it from a message at the top of the screen saying: A chunk of spacecraft has fallen from space and landed nearby, but no envelop icon to the right of the screen.
They are an excellent source of components since once disassembled, they provide 8-9 units. Like other chunks, they can serve as a source of cover.