What's a firmament?

Abraham 4 & Moses 2
What's a firmament?
In the beginning, after God had created the earth, and after He seperated night from day. He set a firmament in the midst of the waters, And yet the firmament was placed above and below the waters. He called this firmament 'Heaven'. He continued on to call forth 'dry land', and the 'Sea'. So there is a firmament called Heaven, there is the dry land called Earth, there is the water below Heaven called Sea, so what is the water above Heaven? Why did God put water over Heaven?