Ye are the salt of the earth - What does it mean?

Ye are the salt of the earth: but if the salt have lost his savour, wherewith shall it be salted? it is thenceforth good for nothing, but to be cast out, and to be trodden under foot of men. Matthew 5:13

Mark 9:50
Salt is good: but if the salt have lost his saltiness, with which will you season it? Have salt in yourselves, and have peace one with another. Mark 9:50

Salt is a preservative, if it has lost it's saltiness it is not good as a preservative or to add taste to food. The Christians are to be salt, to prevent the rotting of society and also to add flavor. A Christian who does not obey Jesus and live in righteousness and condemns the wrong in society, is useless.