掉 (diào) means to fall or drop as a standalone verb. As a result complement attached to other verbs it indicates removal, elimination, or completion: 忘掉 (to forget completely), 扔掉 (to throw away), 去掉 (to remove), 卖掉 (to sell off). This complement use is very common in everyday Mandarin and gives 掉 a sense of getting rid of or finishing off something definitively.