In siman 257, Maran begins to outline the basic principles of the halachot of Hatmana, insulating a pot of food in order to keep it warm. In general, there are two forms of Hatmana. The more problematic form is called Mosif Hevel, where the insulation actually generates heat. The second, more lenient method is referred to as Aino Mosif Hevel, where the insulation merely maintains the heat already present in the pot.
On Shabbat itself, both methods of Hatmana are prohibited. The Hachamim were concerned that if one engages in insulating, his concern for the heat of the pot might bring him to actually heat up the food on a flame, in the event that it already cooled off.
On Erev Shabbat, the laws of Hatmana are more lenient. It is permitted to insulate with a material that does not generate heat, Davar SheAino Mosif Hevel. However, it is prohibited to insulate with Davar SheMosif Hevel, a heat-generating material.
One might ask, what could possibly be wrong with insulating before the start of Shabbat? The answer is that the Hachamim were concerned that one might insulate using Remetz, hot ashes and come to stoke the embers to rekindle the fire.
SUMMARY
1. On Shabbat it is prohibited to insulate a pot with any material.
2. On Erev Shabbat it is permitted to insulate a pot only with a material that does not generate heat.
