When one removes a boiling pan from fire (usually right before Shabbat), the pan is hot. Its heat is considered the extension of the fire itself, and the pan is called "the first degree from fire." This pan can therefore cook the food put into it, as if it were still on the stove. Consequently, one should not put spices into it, because he would then be cooking spices on Shabbat. However, once he serves the food in a plate, the plate is only "second degree from fire," and one can safely add any spices he wants.
Rav Yosef wanted to say that salt has the same law as spices, but Rav Nachman told him, "Salt needs as much cooking as the meat of an ox," that is, salt can be cooked only the hottest of the pots, and it is thus no problem putting salt into a hot pan removed from fire.
Art: Pietro Falca - The Spice-vendor's shop