Celery Salt is ground celery seed mixed with table salt. Commercially, the salt is often actually mixed with an extract made from Celeriac.
The seed used doesn’t come from the celery that we ate, but rather from the ancestor of that celery, which is called “Smallage” or “Wild Celery.”
Celery salt is used for, among other things, Bloody Mary cocktails. It’s used either in the drink mix, or used to rim the glass with, or both.
In cooking, something celery flavoured (celery or lovage or celeriac), and some salt, if you feel the additional salt is needed.