If two items (food or utensils) touch each other, they do not absorb taste from each other without a catalyst. One possible catalyst is heat. There are many different ways to produce heat in halacha; cooking, roasting, baking, and so on. As an aside, this is the reason that we use separate pots for meat and dairy. A pot can absorb the flavor of meat when it is cooking and put it back into a dairy food that was cooked in the pot later on. This can happen even without meat actually being in the pot at the same time as a dairy food.

Not all heat is created equal. At the strongest level of heat, the two foods would mix completely. At a lower level the food might mix in as deep as the width of a finger – a netila, for example when cutting a sharp food with a non-kosher knife. Other times the taste only goes into the very outer layer- a kelipa, for example when pouring a hot liquid onto a cold food. There will be differences in the cooking power of food on the fire, as opposed to food that is hot in a kli rishon (first pot, the one that was actually used on the fire) or kli sheini (second dish, usually a plate or bowl into which food was transferred). For example, a kli rishon on the fire may have the power to cook and transfer taste from a forbidden food to a permitted food, even if it is only a drop warm and not yad soledet bo (the temperature at which an infant gets scalded) (Shach 105 s”k 5). Food in a kli sheini can’t cook, meaning it would not transfer taste strictly speaking, but there will still be some stringencies that will be mentioned later on.

Something that can halachically cook is called cham, meaning hot (yad soledet bo is generally the minimum temperature to be considered hot). Something that cannot halachically cook (even if it is warm) will be referred to as tzonen, cold.