Celery salt is a seasoned salt used as a food seasoning, made from ground seeds, which may come from celery or its relative lovage. It may also be produced using dried celery or seed oleoresin (Wikipedia, nd).
Since celery salt can be traced back to the main ingredient celery, if you are allergic to celery do not consume celery salt (Anaphylaxis Campaign, nd).
Celery salt is an ingredient of the Bloody Mary cocktail and the Caesar cocktail. It is also commonly used to season Chicago-style hot dog, the New York System wiener, salads, and stews. It can also be used to liven up a coleslaw. It is a primary ingredient in Old Bay brand seasoning. Celery salt is often used by food producers to prevent spoilage while avoiding listing sodium nitrate on the ingredients list directly (Wikipedia, nd).