The imperative form of the verb 'to come' (kuru). A colloquial, often forceful command meaning 'come here' or 'come on!'
早く来い、時間がない。
Come quickly — there's no time.
犬を呼んで『来い』と言った。
I called the dog and said, 'Come!'
相手に向かって『来い！』と叫んだ。
He shouted 'Come!' at his opponent.