Since earliest times, salt has been used not only as a preservative, but also as a condiment at the table. It was expensive and usually heavily taxed, and so was treated with great respect. At the tables of the wealthy, salt was kept in ornate salt cellars.
It was customary for there to be two salt cellars at aristocratic dining tables: a large and highly decorated one, to the right of the hostess, and a smaller one at the other end of the table. Those who were seated beyond the small one were "below the salt," and therefore unworthy of notice by their betters.
Very beautiful and very large salt cellars were important table ornaments for the wealthy and the noble in Medieval and Renaissance times. This was particularly true in Italy, where many goldsmiths and artists created these unusual works of art. Some salt came from mines, but most came from the sea, and many salt cellars were shaped like seashells, or even made from real Nautilus shells, and were often crafted in gold, silver, and crystal.
The fear of poison--salt could be laced with arsenic powder--even led to a fashion for enclosed salt cellars in the Renaissance.
You will often see large salt cellars in paintings depicting feasts, and you can see actual examples in many museums, like the lovely Museo degli Argenti in the Boboil Garden complex in Florence.
Of course, the most famous salt cellar, is Benvenuto Cellini's masterpiece shown here. In his autobiography Cellini says that it represents the Terra e Mare (the Earth and Sea), with Neptune, the god of the sea, and Ceres, goddess of the earth, joining together the two sources for salt.
(This most famous piece was stolen from a museum ion Vienna in 2003 and recovered in 2006. You can read an amusing piece about the theft here: Cellini's Stellar Cellar).