I normally don't like the catch phrase "make a difference" but that's the idea Jesus is conveying when he tells his followers they are the salt and light of the world. Salt adds taste to food and light illuminates everything around it. They make a difference. Salt and light are metaphors for the qualities Jesus had just called "blessed" in the previous passage. Your "saltiness" and "light" are the good works you do because you are living for the world to come and are seeking to bring glory to your heavenly Father even now.
I know that it's popular to point out that salt was often used as a preservative in those days and therefore what Jesus means to say is that believers are the preservers of the earth. But I don't know of any other passage that teaches that the purpose of believers' good works is to preserve this world. Furthermore, if Jesus intends salt to be a metaphor for preservation, he should say, "You are the salt of the earth; but if the salt has lost its preserving ability, etc." Instead he says, "You are the salt of the earth; but if the salt has become tasteless..." It seems to me that Jesus speaks of salt as something that adds flavor and taste, not as something that preserves.
Salt doesn't just make food salty the way pepper makes food peppery. Salt is a unique spice because it brings out the full flavor of the food itself; it makes that food taste more like itself. Followers of Christ can salt the earth by showing the world what it means to be fully human, to have the flavor of God's image. The community of believers in Acts was such an example when they shared all things with one another "taking their meals together with gladness and sincerity of heart, praising God, and having favor with all the people" (Acts 2:46-47). That's what humanity ought to look like.
Jesus seems to think that the real obstacle to being salt and light in the world is that we'd be tempted to hide our true qualities. He talks about salt becoming tasteless--as if that were possible. He talks about how silly it is to hide a lighted lamp. Notice that he doesn't speak of these qualities in terms of work and effort. "I'm afraid you're going to slack off in being salty. I'm exhorting you to glow as brightly as you can every day." Instead he speaks of these qualities as if they were the simple truth about ourselves, and he is exhorting us not to be ashamed of them. And when you consider from our previous passage that living as a "blessed" one means going against the grain of this world, conforming yourself to the values of heaven, and risking persecution as a result, you can see why we need Jesus' exhortation not to shrink back from letting that light shine.