[Cyberbob]

Snyder shot about six hours of footage for the film in 2016, combatted with the studio to reduce it further and further for a theatre release, but Snyder ended up stepping down during post-production following the death of his daughter, and Joss Whedon was called in to finish it up.
Joss reshot about 3/4 of the film, and the studio released it with all his reshoots, and only a quarter or so of Snyder's already cut down version.

HBO Max has now paid for Snyder to use his original footage, shoot a little extra, and recut it to his original vision, and release a version which will be drastically different from the version that is 80% Joss Whedon reshoots.

The reception from this Snyder cut will basically form the canon for the DCU moving forward. If it's successful, they'll basically retcon Whedon's version like it never existed.