It's a bit of a riddle. It's asking basically, that "If what you believe to be true is a falsehood spiritually, than how misled are you?" The pharasees and church leaders of the time believed they knew the truth about God and his will, yet they were the absolute furthest from it and more unaccepting and less likely to lead someone to the truth than a pagan outsider (for example, the good samaritan that helped the man in need)

What you think is real is actually the unreal and what seems unreal is the real.This life is unreal because we think we are the body but we are not.What is beauty or color for the blind? The answer is deeper than that. We get attached to our senses when we come in this body. We live this life only with our senses and we get attached to the unreal. Remove all that is connected to the senses and you will understand the truth. The knowers go to this state through meditation on nothingness.Light can be darkness if we do not go through the senses.

OK...so Matthew was actually a tax collector- he was very consumed with darkness as it was always in his profession. He looks into "darkness" in quite a bit of detail. I'll go back to the start of the verse so you can understand the meaning. Matthew 6:22 "The eye is the lamp of the body. So, if your eye is healthy, your whole body will be full of light; (23)but if your eye is unhealthy, your whole body will be full of darkness. If then the light in you is darkness, how great is the darkness!" Lets start with the meaning of eye: the eye is like your heart, what's in your eye; is what you feel. it's the image of your soul- the eye reflects your inside feelings. If inside you're full of light (light= one is full of goodness; inspiration, hope, love, joy etc..) you will reflect this goodness; people can see thru your eyes that your happy, have hope and inspire..but if within you have darkness; hurt, bitterness...etc, this too will be reflected thru your eye; giving an "unhealthy eye" where all can see this darkness. What's meant by "how great is that darkness" is, how much hurt you must have within you, that darkness is what you exhibit. (show) Through your eye. Let me know if you would like further clarification =)

Imagine a room with a window. The window is the eye. The room is the body. If there is nothing obstructing the window then light will shine through clearly into the room. But if something is obstructing the window then there will be darkness in the room.