There are 39 melakhot (creative activities) which should not be done on Shabbat. One of them is cooking(bishul). 'Bishul' includes all related activities, such as: boiling water, baking bread, frying an egg; broiling meat ; cooking a stew, etc.
The prohibition of cooking is independent from the prohibition of lighting a fire or any other source of heat. So, even if you have a fire or another source of heat, active from before Shabbat , you still cannot use it to cook.
The most important distinction you need to bear in mind to understand the rules of 'bishul' is that you cannot cook on Shabbat, but you are allowed to warm up foodon Shabbat.
The Halakha deals extensively with all the casuistics of 'cooking vs. warming' food.
Today I will write a first principle, which is, the source of heat.
The source of heat.
You can't light a fire or activate any other source of heat on Shabbat. But if you have a source of heat activated from before Shabbat, it is permitted to warm food on it. For example, you can use a Shabbat Plata (aka Shabbat hot plate, aka electric blech) to warm upcold cooked rice or a cold
bread on Shabbat.
However, the Rabbis forbade using a live and visiblefire, even if it was lid from before Shabbat, and even when intended to be used just to warm up food (mechaze kemebashel) and not for cooking.
Today, most people would use then a Shabbat Hot plate to warm up food. But before it was invented, people had to use a mecham (heater, with no visible fire); an oven or they had to a cover the live fire with a metal sheet, plate, etc.
Rabbi Yosef Bitton. YMJC 130 Steamboat Rd. Great Neck NY 11024