Coffin nails, the nails used to secure basic pine-box coffins, are items used in the practice of hoodoo/rootwork.
Coffin nails are used primarily for black magic spells against one's enemies but can also be used in white magic spells for protection and to carve names, phrases, and symbols into candles.
In older days, practitioners would dig up coffins in order to retrieve the nails. Since grave desecration is a big no-no and since few people are buried in pine-box coffins these days, an acceptable substitution is rusted nails. The coffin nails that one can purchase at occult shops are in fact rusted nails.