Why is the stove covered with a sheet of hard aluminum on Shabbat?

There are two reasons why the stove-top is covered on Shabbat. Obviously, this covering is only necessary if the stove is ignited in order to keep food warm for the Shabbat meals. Many Judaica stores (and many hardware stores situated in religious communities) sell custom made blechs ("blech" is the Yiddish word for sheet-metal) that will fit your stove. If you don't have access to such a store, any piece of sheet-metal will do.

1. Biblical law permits one to leave uncooked food on a fire before Shabbat, although it will cook on the Shabbat. WE are commanded to rest on Shabbat -- not our stoves. However, the Sages were concerned that one will put food on the fire before Shabbat, and after the Shabbat enters the person might see that the fire isn't adequate, and the food won't be ready in time for the guests... And we all know that this can be an embarrassing situation. This might put cause the person to forget about Shabbat and inadvertently increase the flame -- a real no-no. Therefore the Rabbis instituted that food should only be placed on a covered flame before Shabbat. The covering will remind the person that it is Shabbat and therefore forbidden to turn up the flame.

[This rule only applies if the food is rawish when Shabbat enters. If, however, the food is already halfway cooked before Shabbat enters, or if it is completely raw, it is permitted to leave it on an uncovered flame.]

2. In deference to the prohibition against cooking on the Shabbat, the Sages banned placing even a fully cooked and heated food item onto a flame on Shabbat. A pot may be returned onto the flame on Shabbat only if the flame is covered. (See Is it permitted to warm up a fully-cooked item on Shabbat?)

Truth to be told, if all your food is already cooked (or completely raw) before Shabbat, and you don't plan on returning any pots onto the stove on Shabbat, then a blech is unnecessary. However, it is a universal Jewish custom to place onto a blech all foods which will be warming on a stove on Shabbat. It's one of those thing which give the house a Shabbat feel!