Salt was once a highly valued commodity, much like all the other spices. Because they were rare, all such seasonings were used sparingly by the poor and a sign of wealth for the rich. The superstisions regarding salt, in particular, range from warding off witches to proving a young girl's virginity. In this case, passing the salt or salting someone else's food was akin to cursing them. I know, it seems so ridiculous today; however, in older times, when things didn't always have scientific explanations like we have now, people felt the need to do and believe such superstitions. I presume you may be asking becuase you're reading the poem by this title. If not, you might be interested in reading it. I've posted the link below.
The proverb "Help me to salt, help me to sorrow" has to do with the superstitions regarding salt and luck. It is considered unlucky to give salt to anyone, and superstitious people would either throw salt over their shoulders or offer it to another person to suggest a course of unluckiness. So the proverb literally means that if you offer salt to a person (particularly at the dinner table), then you are wishing bad luck upon that person. There are many cultures in which it is quite normal and acceptable to offer others salt at the dinner table, so keep in mind that proverbs are bound by culture, time, and place and may not be believed and/or practiced universally.