Shabbos, Chapter 14, Mishnah 2. One of the labors forbidden on Shabbos is tanning an animal's hide, and one of the stages of the process was salting the hides - this is tanning. By Torah law there is no tanning with foods, meaning that the labor of tanning does not apply to food, but the Sages decreed a prohibition of tanning with foods: performing on a food that is customarily salted the very same action that was done to a hide. Salting food in large quantities, of the sort used in preparing hides, is forbidden rabbinically, because it resembles the salting and preparing of a hide.
The words of the Mishnah:
"Ein osin hilmi b'Shabbos" - because of this prohibition one may not prepare on Shabbos a large, strong mixture of salt water, since it looks like performing the act of tanning on foods. Although this is not forbidden by Torah law, the Sages forbade it, and therefore one does not prepare the large mixture.
"Aval oseh hu es mei hamelach, v'toveil bahen pito, v'nosein l'soch hatavshil" - a small quantity of salt water may be prepared, and in it one may dip bread or add it to a cooked dish.
The view of Rabbi Yose:
Rabbi Yose disagrees and says: "V'halo hu hilmi bein merubeh bein mu'at" - the name 'hilmi' is indeed used only for a large quantity, but there is a concern here: if we say that the large measure is forbidden and the small measure is permitted, a person might come to say that in a labor forbidden rabbinically there is likewise such a distinction - that in a large measure it is forbidden and in a small measure it is permitted, whereas we know that no such distinction exists. Rabbi Yose therefore disagrees with the ruling that permits preparing a small quantity of salt water.
These are the permitted salt waters:
Nevertheless, Rabbi Yose says that there is a permitted way: when one does something to the salt water that weakens the power of the salt, so that it will not be effective in the tanning process. And this is done with oil - "Nosein shemen batechilah l'soch hamayim o l'soch hamelach" - in two ways:
He puts the oil into the water before he puts the salt into it.
He puts the oil into the salt before he puts it into the water.
In this way the salt's ability to act is chemically minimized by the oil, and therefore there is no problem in salting the vegetables. For this reason even making a large quantity of salt water is permitted, as long as there is oil in the mixture.
In summary: In this Mishnah we learned that because of the decree of tanning with foods one may not make hilmi - a large mixture of salt water - on Shabbos, but one may prepare a small quantity and dip bread in it or add it to a cooked dish. Rabbi Yose forbids even a small quantity, out of concern that people will come to distinguish between a large measure and a small measure in rabbinic labors, and he permits it only when oil is placed in the water or in the salt first, for then the power of the salt is weakened and there is no concern of tanning - and even a large quantity is then permitted.