Before Jesus tells his disciples what salt does, he tells them what it is being sent into. He does not say you are salt. He says you are the salt of the earth. That word earth is not incidental. It is the whole field of play. Not the salt of the church. Not the salt of the Christian community. The whole earth, with all its rot and darkness, is the place to which these people are being sent.
Salt did many things in the ancient world. Scholars have identified over a dozen uses: flavoring, preservation, sacrifice, covenant ratification, fertilizer in small quantities. Most commentators agree that preservation is the primary image Jesus is drawing on here. The world is decaying, and his people are the arresting agent. They slow the rot. They cannot stop it entirely, but their presence in the world makes a measurable difference against the drift toward corruption.
Jay Adams captures the meaning well: Christians are the ones who make the earth palatable. Life alienated from God is drab, dull, and flat, like food that needs to be salted to bring out its flavor. The people who carry the gospel and have been transformed by grace are the ones who bring that flavor back to a world that has lost its taste for the things of God. This is not a passive existence. It is a presence that does something to everything it touches.
Notice also that moral decay is not sudden. It is incremental. Food does not go from fresh to rotten in an instant. It takes time, and the decay is quiet. Jesus is calling his people to be present in the slow places, the workplaces, the neighborhoods, the family tables, the places where rot happens gradually and where salt can do the most good precisely because no one else thinks to look there.