Salt is a rather mundane object now, but back in the day it was used to keep food from spoiling and was a greatly needed commodity. Salt is not only great for a bit of seasoning or preservation, it is also a spiritual tool. The use of salt for preservation has made it a symbol of life and works against impure and unclean spirits. One of the most common uses of salt in spiritual practice is to form a boundary with it. A line of salt across the path of a doorway or a windowsill will bar entry to unwanted spirits. You can also draw a salt circle for protection. However, drawing a salt circle or laying salt lines can only be effective in a "clean" house.
Your house must be physically and spiritually clean for salt barriers to work. If you lay one without knowing what spirits are within or know for certain the house is clean you could very well trap the entity within the house/ area. Breaking the line or circle makes the barrier null and void. The salt must be consistent throughout the barrier.
We "buy" our salt from Our Lady of Lebanon, a Catholic national shrine.