？
Are you free after school?
Literal
After-school [topic-は] free-[ongoing-ている] [question-か]?
空いている (lit. 'is empty/vacant') is used idiomatically for 'to be free, have time available' when applied to a time slot or person's schedule. Contracted to 空いてる in casual speech — the い of ている is dropped. The final か marks the question.