# You are the salt of the earth

"You are like salt for the people of the world" or 2) just as salt preserves food, disciples of Jesus keep people from becoming totally corrupt."

# if the salt has lost its taste

Possible meanings are 1) "if the salt has lost its power to do things that salt does" or 2) "if the salt has lost its flavor."

# how can it be made salty again?

"it cannot become useful again." 

# except to be thrown out and trampled under people's feet

"except for people to throw it out into the road and walk on it"