"You are the light of the world. A city set on a hill cannot be hidden. Nor do people light a lamp and put it under a basket, but on a stand, and it gives light to all in the house. In the same way, let your light shine before others, so that they may see your good works and give glory to your Father who is in heaven." (Matthew 5:14-16 ESV)
There is a similar passage in Luke, but this passage seems to be talking about something entirely different.
"No one after lighting a lamp puts it in a cellar or under a basket, but on a stand, so that those who enter may see the light. Your eye is the lamp of your body. When your eye is healthy, your whole body is full of light, but when it is bad, your body is full of darkness. Therefore be careful lest the light in you be darkness." Luke 33-35 ESV
This passage follows Jesus' talk about how the current generation is evil because it keeps asking for a sign, but no sign will be given to it except the sign of Jonah, which was Christ's death and resurrection—a pretty obvious sign if you ask me. Jesus also says that the queen of the south who came to hear Solomon will condemn this generation because she heard and believed Solomon's wisdom, and now someone greater than Solomon was present, that was Jesus, God-in-the-flesh.
Christ was the lamp put on a stand so all could see. God did not hide him or put him under a basket. And yet, plenty of people missed him. Why? Because their own "lamps", their eyes, were unhealthy—not their physical eyes, but their spiritual eyes, their ability to discern God's presence in this world.
If our spiritual eyes are no good, no light gets in, i.e., if we cannot discern how the lamp on a stand (Christ's life and death and resurrection) shines light into our rooms (enlightens every aspect of our lives), we shall have spiritually dark hearts.
So what's the admonishment? "Be careful lest the light in you be darkness," or, as the Good News Translation says, "Make certain, then, that the light in you is not darkness. If your whole body is full of light, with no part of it in darkness, it will be bright all over, as when a lamp shines on you with its brightness." (Luke 11:35-36 GNT)
In other words, if your spiritual eyes really see Christ, he will shine into every corner of your life. There will be no argument, no relationship, no pet hobby, no alone-time, no personality quirk, no meal-prep or mundane job that isn't baptized in his light. Either we see how Christ's death and resurrection affects every part of us or we live in spiritual darkness.
Wouldn't that be a different meaning to the song "This Little Light of Mine"?
This little light of mine
I'm gonna let it shine.
The light of Christ into me
I'm gonna let it refine.
The work of the cross upon me
I'm gonna see how it unbinds.
Let him unbind, let him refine, let him shine.