Light makes everything bright and clear. The light meant here is not a cold light. It is the glowing light of hearth and lamp, the light that shines from a ring of torches or that streams from the windows of houses where community is alive. It is the light of truth that exists in love, and the love that rejoices in truth, justice, and purity. It is the love that lightens up faith, that brings clarity to everything.
Light is like salt because salt consumes itself, just as a candle burns itself down.
In the Sermon on the Mount, which is a proclamation of love, Jesus speaks of adultery, which can also consist of thoughts and feelings of the heart. Adultery breaks a relationship of faithfulness, truthfulness, and responsible love. But light and salt overcome such things. The same is true when people swear oaths and make vows in order to be believed. Say simply yes or no.