洗劫一空 is a four-character idiom (成语) meaning 'to be completely looted or robbed of everything'. It describes a situation where thieves or invaders take all valuables, leaving nothing behind. The literal breakdown is 洗劫 'to loot' + 一空 'completely empty'. You can use it to describe a robbery, a natural disaster that destroys everything, or even metaphorically (e.g., a store being cleared out by shoppers).