how can you be salt for the earth and light for the world?

Using the metaphors, salt and light, Jesus challenges us as to our role in bringing about the Kingdom of God. I discovered that salt can bring about healing when I used salt water dealing with a sore tooth.  I know that I must use salt to bring flavor to the food I prepare. I also know that if I use a low wattage light bulb, I cannot see adequately.  What do these metaphors of salt and light mean? 
 
When we pay attention to the needs of others and reach out to them we can bring about healing.  We can bring a new flavor to them as they react to our attention.  The light of love that I share with others can help them to glorify God as they come to know God more deeply.  Jesus expects us to be active in our families, in our neighborhoods and in the greater world.  He expects that our commitment to his way will bring about healing.  It will bring about a world that is filled with the goodness and joy that comes from adequate light through acts of love and mercy.