"people can clearly see everything that comes into the light." The Bible often speaks of God's truth as if it were light that could reveal the character of something.
Possible meanings are 1) Paul is addressing unbelievers who need to wake up from being dead spiritually just as a person who has died must come alive again in order to respond, or 2) Paul is addressing the Ephesian believers and using death as a metaphor for their spiritual weakness. (Isaiah 26:19, 51:17 and 52:1)
Christ will enable an unbeliever to understand how evil his deeds are and how Christ will forgive him and give him new life, just as light shows what is being hid by darkness.