The super flash is a graphical effect that appears when certain moves (particularly super special moves) are performed. In general, this is a literal flash or spark appearing on, around, or near a character. In many cases, this is accompanied by a temporary change of background scenery (usually, but not always, something dark).
Also, at this point, if the move expends meter, this is usually when it gets spent.
During this period, lots of games will temporarily suspend all movement; in some games, this gives the other person an opportunity to make split-second decisions, while in other games, input becomes ineffective, possibly causing a misfire.
Because everything is halted, nobody can take an attack during this time.
In the Guilty Gear games, the equivalent of the super flash for supers and Force Breaks do not halt projectile movement, nor does it halt their attack hitboxes. Because characters are still unable to move, unless one inputs a block before the halt, projectiles become unblockable.
And depending on the character, it may be possible for this to continue into a combo via the move that caused the halt in the first place. This is comparatively easy with Dizzy’s Ki-no-Mi wo Toru Toki ni Tsukatteta n desu into her Imperial Ray.