7 And God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament; and it was so.
8 And God called the firmament Heaven. And the evening and the morning were the second day.
On this day, God made the firmament. This word is H7549 רקיע (râqîya‛ raw-kee'-ah. From H7554; properly an expanse, that is, the firmament or (apparently) visible arch of the sky: - firmament.) according to Strong's Concordance.
Remember that the entire earth was covered with water at this point and God calls it the deep (v2). God here divided the waters in such a way that there is a firmament or an expanse of a sky in between the waters.