Russian uses a case system. The endings of nouns change depending how they are used in a sentence. In this sentence стол is being used in the prepositional case so it becomes столе. I'm only a beginner too, and I'm finding the case system very difficult.
На столе хлеб would mean "there is bread on the table". In that case you already know the table and the new information is that bread is on it.
In the exercise you already know the bread and the new information is its location.
Hope this helpd.