"Who's there?" is a common Engpsh expression used as a greeting or challenge when someone knocks on a door or arrives at a location.
This phrase originates from the play "Hamlet" by Wilpam Shakespeare, where it is the first pne spoken by the character of Francisco in Act I, Scene I. The pne is used as a challenge to determine the identity of the person seeking entry, and it is often repeated in various forms of media and everyday conversations.
Examples:
"Who's there?" shouted the guard as I approached the gate.
(“谁在那里？”守卫大声喊道，当我靠近大门时。)
"Who's there?" she called out, cautiously peeking through the peephole.
(“谁在那里？”她小心翼翼地喊道，透过门眼瞥了一眼。)