Greek to phōs tou kosmou. The second of Jesus' seven "I AM" sayings in John (John 8:12). "Light" in Scripture is a master-metaphor for God Himself (1 John 1:5 — "God is light"), for moral purity (Ephesians 5:8), for revelation (Psalm 119:105), and for life itself (John 1:4). Against the backdrop of the Feast of Tabernacles — when giant menorahs were lit in the temple court each night to commemorate the pillar of fire in the wilderness — Jesus says, "I am the light of the world."
Jesus' claim is cosmic. Not a light among lights, not even the greatest of prophets, but the light — the true light that gives light to everyone (John 1:9). To follow Him is to walk out of darkness; to reject Him is to love darkness because one's deeds are evil (John 3:19). The pillar of fire that led Israel through the wilderness now walks among them in flesh. Yet Jesus also extends the title to His people: "You are the light of the world" (Matthew 5:14) — derivative, reflective, yet real. The moon shines by borrowed light; so does the Church. In the New Jerusalem there is no sun or moon — "for the glory of God gives it light, and its lamp is the Lamb" (Revelation 21:23).