What does Catholic Church teach about the interpretation of the metaphor of "salt of the earth?"

We see Jesus telling at Matthew 5:13 (NRSVCE): "You are the salt of the earth; but if salt has lost its taste, how can its saltiness be restored? It is no longer good for anything, but is thrown out and trampled under foot." My question is: what exactly is the significance which Jesus is attributing to the earth with reference to salt . Does he mean that the earth is the source of salt ; or does he mean that salt is really valuable - both as a taste-maker and as a preservative-- for mankind which dwells on the earth? What do the teachings of Catholic say about the interpretation of the metaphor or "salt of the earth?"