What is the meaning of εἶ?

Old name of the letter ε (epsilon, see ἒ ψιλόν (è psilón), έψιλον).

second-person singular present indicative of εἰμί (eimí): thou art.

second-person singular present indicative with future meaning of εἶμι (eîmi): thou wilt go.

Source: wiktionary.org