What does this parable mean? There are several ways to look at this analogy, and they are all deeply meaningful. Salt and light both enhance everything around them. They give of themselves to make the things around them better, meaning that they both make the things they come into contact with more like that thing. Salt makes the food it is on taste more like that food. Light makes a thing more clearly visible, and not hidden in darkness.
Salt and light also have the ability to purify. When meat is salted, the salt preserves the food, preventing bacteria, and other things from corrupting and rotting the food. Light as well has similar properties. Dark and wet places grow bacteria, and disease, but places that are in direct sunlight are the opposite, they are dry and sterile. With enough time, anything placed in the light will be purified.
Finally, Salt and light both have a spiritual warfare aspect as well. Conquering armies would use salt to kill farm fields of their enemies, and it has always been used as a sacramental that can be used for protection from evil spirits. So too with light, Consider the metaphor of the cockroaches scurrying away when the lights come on. The devil and his demons are just the same, they love to live in the darkness and shadows, and they hate light, they hate having their ugliness exposed for all to see.
So today Jesus is saying that we have to be like salt and light in the world. We have to enhance the world around us, shining light on it, and making it better. We have to use our nature and our connection with Jesus to purify those that we come into contact with every day. And we have to fight off the evil in our midst, conquering our worlds, claiming them for christ, and making them intolerable places for the evil forces of the world to ever return. It may seem like a tall order to fulfill, but with God all things are possible. So go out into the world and be salt and light.
Oh lord, help me today to be salt and light to the world, helping to enhance those around me, to purify those that you have given me to work on, and give me the courage to fight evil and send it off into the darkness.