The Flashbang was a grenade that functioned like the Smoke Bomb. It was composed of smoke-like gas that temporarily turned a victim blind and deaf, and thereby left them open for attacks.
Batman: Arkham Origins Incident
Flashbangs were used by Deathstroke during his attempt to end Batman on the Final Offer. After he had enough punishment, Deathstroke threw a Flashbang to temporarily blind Batman, and then attacked him with multiple strikes in quick succession. When playing as Deathstroke, they were called Stun Grenades, and worked identically as Batman's Smoke Bomb.
Batman: Arkham Knight Incident
They were utilized by Jason Todd during his boss fight in the Militia Headquaters, and were later used by him during his attack against Black Mask. These were much smaller in size and only affected one enemy. They acted as a ranged stun move, which allowed Jason to perform Beatdowns on affected enemies. If close enough, Jason would use a regular Beatdown, but if far enough, he used his guns instead.