Why is the Challah dipped in salt before it is eaten?

We always dip the bread in salt, not only on Shabbat.

Our table is considered an altar (see Ezekiel 41:22 and Ethics of our Fathers 3:3), and in the Holy Temple salt was offered together with every sacrifice (Leviticus 2:13).

Salt never spoils or decays, therefore, it is symbolic of our eternal covenant with G-d. That's why the verse refers to it as "the salt of your G-d's covenant."

[Salt also adds taste to everything. Our bond with G-d is supposed to add meaning and flavor to every moment of our lives; even when we are not directly involved in spiritual pursuits.]