A simple dragon is a dragon of only 1 element. Because there are only 10 elements, there are only 10 simple dragons, named after their element. These dragons are the fire dragon, the plant dragon, the earth dragon, the cold dragon, the lightning dragon, the water dragon, the air dragon, the metal dragon, the light dragon, and the dark dragon.
When you breed two of the same simple dragon breeds, you will get a third dragon of that breed. If you breed two simple dragons of different elements, or 2 different dragons, you will get a dragon hybrid.