The eye is the lamp of the body. If your eyes are good, your whole body will be full of light.
Remember that song we sang as kids, "Be careful little eyes what you see." This is exactly what this verse is talking about. If you keep your eyes viewing what is good in his site then you will be full of His light. How do you do that then, and see those that need help, that need love..... the poor, the dirty, the sinners? Because everything is viewed through Christ. If I view these things through Him, then I see the wonder that he can do in a life... fill it full of light. If I shine the light into darkness, there is no more darkness. But I have to beware to always look at the light that He is shining.
Be careful little eyes what you see.