This is the 348th most frequent Ukrainian word.
хтось
"Someone" or "somebody."
The word 'хтось' is used as a subject in this sentence, implying an unspecified person responsible for the action.
Хтось залишив двері відкритими.
Someone left the door open.
Here, 'хтось' indicates an unknown individual performing the action. It functions as a nondeterminate subject.
Я чув, що хтось співає поруч.
I heard someone singing nearby.
In this question, 'хтось' is used to inquire about an unspecified person who fits the described criteria.