What does Matthew 5:15 mean?

Nights could be very dark during the time of Jesus. In fact, to modern eyes living in a time of electricity, the inky nighttime darkness of the ancient world would likely be shocking. On a moonless or overcast night, people would have struggled to see their hand in front of their own eyes. Caves and interiors of large rooms would have been naturally devoid of any light.

Light was available in the form of fire, including oil lamps. As houses grew dark after sunset, lamps would be lit and, if available, distributed around the house. Placement was key. The lamps would be put on a stand in the optimal spot to provide as much light to the room as possible. This is the point of Jesus' comment in this verse: why would anyone light a lamp in the nighttime and then put a basket over it? They would not, Jesus says. The light of lamps is meant to be seen in the same way that the light of Christ is meant to be seen in the world.

By way of analogy, believers in Jesus are the lamps. In fact, they are the only source of true spiritual light, the light of Christ. That light is meant to be seen, so Jesus tells His disciples not to hide it or cover it up for any reason. The following verse is a famous expression of the need for Christian goodness to be visible to the world. Doing the works given by God is how His disciples will distribute His light. Failing to do those good works, then, is like putting a cover over the only lamp in a dark room; hiding the light makes the lamp useless.