What a bold statement!  Jesus claimed to be the only light in a dark world (John 1:1-5).  His Jewish listeners would have remembered Genesis 1:3-4: "And God said, "Let there be light," and there was light.  God saw that the light was good, and he separated the light from the darkness."  They knew that God is the only true source of physical, moral and spiritual light, and that without God the world is naturally in darkness - physically and spiritually.
 
Jesus identified Himself with God.  He used the 'I AM' name of God from the Old Testament (Exodus 3:14) and then said that He has the power to hold back the darkness.  Although He was speaking spiritually, darkness physically replaced the midday sun for three hours when He was dying on the cross while bearing our sin (Luke 23:44-45).  But Jesus was saying that the world is a spiritually dark place full of people who are unable to see the truth or relate to God.  That is why Jesus came, so that those who believe in Him will be able to see the glory of God, know where they are going, and will live with confidence and safety.
 
The message is stark. In this dark world, everybody needs Jesus otherwise all will stay in the dark - not knowing who they are, where they are or where they are going.  Without Jesus nobody is able to understand what is true about God and eternity.  But those who trust in Him can see how to live because Jesus walks with them.  They know they are safe; they know their life's purpose and are confident of eternal life with Him.  They can live pleasing God because He shows them how to do it. His example, His Word and His presence go ahead of them every day.

This is a wonderful message full of confidence and joyful hope: it is the message which so many of our friends and colleagues need.  So, ask the Lord to get you ready to tell them!  Their darkness need not last any longer; as they repent and turn to Jesus they can live unashamed in the light.