It's a graphic novel set in the DC universe. It likely involves the superheroes and villains of DC in a story filled with action, drama, and moral dilemmas. Maybe it shows an alternate version of the DC world where justice is perverted in some way.
The 'dc comics injustice graphic novel' generally revolves around a world where Superman has gone bad. He starts to enforce his own version of justice in a very extreme way. This leads to a big divide among the superheroes, some siding with Superman's new regime and others opposing it.
As of now, the current Flash is usually considered to be Barry Allen. But the DC Comics universe is complex, and there could be alternate versions or storylines that feature other characters as the Flash temporarily.