Shabbos, Chapter 3, Mishnah 5:
"HaMeicham shepinahu - lo yitein letocho tzonein bishvil sheyeichamu" - "A kettle that one removed from the fire: he may not put cold water into it so that the water will become hot."
A "meicham" is a vessel containing hot water, which was resting on a stove, and "shepinahu" means that he removed it from the stove and from the fire. Even though it was taken off the fire, it still retains the status of a keli rishon (a first vessel), and therefore one may not put cold water into it in order to heat that water, for in doing so he would be cooking the cold water on Shabbos.
"Aval nosein hu letocho o letoch hakos kedei lehafshiran" - "But he may put water into it, or into a cup, in order to take the chill off it."
"Letocho" - one may put cold water into the meicham itself only in order to take the chill off it, and not in order to heat it to the point of actual cooking. This is accomplished by putting in a large quantity of cold water: a small amount of water would be heated by the heat of the water in the meicham, whereas a large amount of water poured into very hot water merely takes the chill off itself and is not cooked by it.
"O letoch hakos" - the cup is a keli sheini, that is, a vessel into which the water was poured from the first vessel that had stood on the fire. A keli sheini, by its very definition, does not cook what is placed into it but only makes it lukewarm, at least as far as halachah is concerned.
"Ha'ilpas vehakedeirah shehe'eviran meirosechin - lo yitein letochan tavlin" - "A stew pan and a pot that he removed from the fire while they were boiling: he may not put spices into them."
The ilpas and the kedeirah are two types of vessels that were used for cooking on the stove. When one removed them from the fire while they were actually boiling, one may not put spices into them as long as they are hot, since the spices would be cooked in them.
"Aval nosein hu letoch hake'arah o letoch hatamchui" - "But he may put spices into a bowl or into a serving dish."
If the food was transferred to a bowl or to a serving dish, which are a keli sheini, one may put spices into them. According to the tanna kama, a keli sheini - in which one does not cook to begin with, but into which the food was poured - is not hot enough to cause cooking, and therefore one may put the spices into it.
The view of Rabbi Yehudah:
Rabbi Yehudah disagrees and says: "Lakol hu nosein, chutz midavar sheyeish bo chometz vetzir" - in his opinion one may put spices into any vessel, even into a keli rishon, into the very stew pan and pot mentioned above in which the food was actually cooked on the stove, since a vessel that is no longer standing on the fire has no power to cook. He does agree, however, that something containing vinegar or brine is different: their sharpness assists the cooking process, and therefore they do have the power to cook.
In summary: in this Mishnah we learned the distinction between a keli rishon and a keli sheini. A kettle and a pot that were removed from the fire retain the status of a keli rishon and still cook, and therefore one may not put cold water into them in order to heat it, nor may one put spices into them; but putting in a large quantity of water merely to take the chill off it, or putting spices into a cup, a bowl or a serving dish, which are a keli sheini, is permitted, since a keli sheini does not cook. Rabbi Yehudah is lenient and permits putting spices even into a keli rishon that is no longer on the fire, excluding only something containing vinegar or brine, whose sharpness assists the cooking.