Biblically it is a divider, described as a massive tent built of crystal with a pale hue, that separates the waters of the heavens (above us) from the waters of the deep (below us). The vaults of the heavens and deep were opened once in the bible and it flooded the entire world (noah’s flood).

It’s all in the bible if you care to read it (and supposedly werner von braun has a specific passage about it on his gravestone). Alternatively, you can deduce/derive the firmaments actual existence (or something that serves its purpose in any case) by scientific law.