Salt is part of what Princess Peach used to bake a cake for Gourmet Guy in the event of Paper Mario so that she could get Mario the info he needed to know where the next Star Spirit was being held. It was only a minor item, and didn't appear again. Salt was on the wooden table in the back of the kitchen of Princess Peach's Castle. It was placed in the middle of Sugar and Eggs.
Salt is based upon the real-world substance, sodium chloride, which also goes by the name of Salt.