What is the firmament mentioned in Genesis?

The word firmament means a great expanse.  When God created a firmament in Genesis 1:6-8 He was creating what we call the universe today.  He made a space that He would be able to fill with everything that we know in the universe.  He called the firmament Heaven at this point and we still call outer space the heavens today (Psalm 19:1).  But there are different levels of the firmament.  The birds fly in the firmament just above the earth (Genesis 1:20) and the stars are in the firmament beyond our reach (Genesis 1:14).  The idea of the firmament is easy to understand, as long as we search it out.