Exposition:
The world in which we live, has chosen darkness rather than light (John 3:19). This darkness is a spiritual darkness. Just as you are liable to fall and hurt yourself in natural darkness, just as you are open to accidents when you drive with no headlights or lamplights, so it is in a world full of darkness.
The dark state of this world has caused many to fall into avoidable sins that have sent them into the wilderness of sin. The Lord now says you are the light of the world. To be light means in times of darkness, you offer hope and lead the way; you are the luminary, the guide, the hope of this dark world and the deliverer of people from darkness.
Jesus Christ, our model, being the light of the world, showed forth His life through His sinless life and won many out of darkness to Himself. As the light of the world, let us put on the very nature of Christ and shine forth our Godly character to dispel darkness wherever we find ourselves. (1Peter 2:9) God takes obscure men, dignifies them with His Character and unleash them on display. As we have been dignified, we should do what light does; shining for the ungodly to see our godly character in this dark and perverse world.