Salt is not only a preservative but can also be used to disinfect wounds. Christians should disinfect society from selfishness, immorality, drug abuse, depression and so much more. The only way to do that is by sharing the love of Jesus Christ with people all around us, and to stand up for what is right according to God's Word and His plan.
If you put a teaspoon salt in a jug of water, every single drop of water in that jug will have the taste of salt. We as Christians should live our lives in a way that every single person will get to know the Lord Jesus. We should affect all aspects of society.
We can have a downwards influence to those 'under' us - gardener, employee, student etc. Then we can influence those 'next to' us - spouse, colleague, friend, sibling, neighbour etc. We can aso have an influence 'upwards' - a parent, manager, teacher, officer etc.