The Gemara in Chullin (97b) says that a salted food is considered as if it is hot. In order for food to be considered salty enough that it is as if it were hot, the food must be too salty to eat. This level of saltiness, Rashi explains, is not that it is inedible, rather it is unpleasant to eat. Rabbeinu Tam explains that this means the amount of salt one puts on meat when salting it to kosher it.
The Shulchan Aruch (יו”ד קה:ט) rules the same as Rabbeinu Tam, that anything with enough salt for koshering is considered hot. The Rama says that we don’t know exactly how much salt this is, therefore anything that is salted as one would salt meat before koshering through roasting is also considered hot. (Since when one is koshering through roasting, salting is the final step before eating that piece of meat, the amount of salt used is significantly less than when salting meat for the normal koshering process. Therefore, a food that is somewhat salty can be considered to be hot according to the Rama. Whenever a question regarding salting arises, one should consult a rabbi.)
The Ran says that the salted food is only considered hot after the amount of time it takes to get food hot on a fire, about 18 minutes. However, most Rishonim agree that salted food is considered hot immediately. The Shach (יו”ד צא ס”ק יא) rules the same as the Rishonim that say that the food is considered hot right away, and adds that one must be stringent even in a case of great loss.
The Shulchan Aruch explains three levels of how far taste travels from a salted food. If the salted food was lean, the taste travels into the next food only a kelipa. If it was a bit fatty, then it travels a netila. If the food that was salted was really fatty (the Shulchan Aruch says that it was literally fat), then the taste travels through the entire piece of food that the fat touches. The Rama writes that we do not know what is considered fatty or lean, therefore if salted food which is forbidden touches permitted food, one needs sixty times more permitted food than forbidden food to permit the mixture. Additionally, one must take off a kelipa of the permitted food in case the forbidden food was not really fatty and only went into the outer layer of the permitted food and was not really nullified. (The Rama doesn’t write to take off a netila at any time.)
The halachot of salted food are generally referring to a forbidden food that is salted which touches permitted food that is not salted. The reason is that salted food is “as if” it is hot, but does not have the exact same rules as hot food. Therefore, salted food can give off taste, but will not absorb taste from an unsalted food that it touches, unlike food that is actually hot. If so, when permitted food is salted and touches unsalted forbidden food, it would not absorb taste from the forbidden food at all. The Rama (יו”ד קה:י) says though that one should be stringent if, by doing so they will not incur a big loss.
