"'You are the salt of the earth. But if the salt loses its saltiness, how can it be made salty again? It is no longer good for anything, except to be thrown out and trampled underfoot.'" NIV translation
Most of the salt that is used in Israel comes from the Dead Sea but the Dead Sea also contains many other minerals. These other minerals (impurities) cause the salt to lose some of its saltiness and so, the people would have known what Jesus was talking about. Salt is used to add flavor to foods and for preservation and Jesus was reminding His disciples that they were receiving the power to preserve the people from death and decay. If they chose to not do so or to add some of the Law of Moses into the Gospel, then, they would be of no use to the kingdom. As His disciples in the world today, the same thing applies to us. The pure salt is the Gospel of the grace of Jesus Christ.
"'You are the light of the world. A town built on a hill cannot be hidden. Neither do people light a lamp and put it under a bowl. Instead they put it on its stand, and it gives light to everyone in the house. In the same way, let your light shine before others, that they may see your good deeds and glorify your Father in heaven.'" NIV translation
When you try to keep light for yourself by hiding it, it stops being light. In Jesus' day, they used small clay lamps that burned olive oil. If you lit that lamp and then put a bowl over the top of it, it would quickly go out.
Each and every day, God gives us opportunities to let our light shine by telling people of the grace of Jesus Christ. If we make the most of those divine appointments, people will accept Jesus Christ and God will be glorified. If we try to be closet Christians or pew-sitters, then, our own fire will dwindle and go out. We, like the unsalty salt, will be of no use to the kingdom and will be trampled by this world.