In the Ancient Near East, the custom was to seal a contract by bringing salt and eating it with bread. When a person offered up a meal-offering to Hashem, it was as if he was making a covenant with Hashem. Alternatively, salt is a unique substance. It not only gives a food taste, but also preserves it. In preserving food, salt seals it off from the outside forces which try to change and decay it.
Salt thus represents immutability.
For this reason, the juxtaposition of salt with Hashem's covenant reflects the nature of the Torah, His covenant with the Jewish people. The Torah is everlasting and unchanging, exactly the same today as it was when it was first given. Accordingly, when a person wants to come closer to Hashem by offering Him his material possessions, Hashem requires that he include salt, the symbol of the Torah's immutability. If your meal-offering will be prepared in a deep pot, it shall be made of fine flour with oil. Each offering corresponds to a different type of person.