SPOILER WARNING: This article contains major spoilers for "The Race of His Life," "The Flash" Season 2 finale.
In "The Race of His Life," the Season 2 finale, "The Flash" revealed the identity of the man in the mask: he is none other than the true Jay Garrick, who is also Henry Allen's doppelganger.
As it turns out, this Jay Garrick wasn't from Earth-2, as many fans theorized; instead, he comes from another world entirely, a universe he labeled Earth-3. Zoom attempted to steal his speed as he did Barry's but never quite succeeded, so he trapped the true Garrick in order to keep him as a sort of trophy.
At the tail end of the episode, Cisco and Harrison Wells very carefully removed the mask, revealing Jay's true identity. Soon after, he donned a bright red-and-blue costume reminiscent of his comic book counterpart and nabbed Zoom's hero helmet while he was at it, though he added that it had never once belonged to him. After thanking Team Flash, he took Harrison and Jesse back to their own universes through a portal to Earth-2 generated by Cisco.
Jay Garrick wasn't the only huge reveal of the episode; at the very end, Barry also sped back to the night his mother died -- only this time, he stopped Reverse Flash from killing his mother, irrevocably changing his past. This mirrors the events of Geoff Johns and Andy Kubert's "Flashpoint," which saw Barry do the same to disastrous effect; after Barry changed his own personal history in this comic event, he not only lost his powers but encountered a bleak world where Thomas Wayne became Batman and Superman was detained by the government his whole life, among other things.
The episode also referenced another major comic moment during Zoom's demise. When the time wraiths came for Zoom, he disintegrated at their touch; as they pulled him into the Speed Force, a glimpse of his decaying face made the character appear like Black Flash. Created by Grant Morrison, Mark Millar and Ron Wagner, this character often heralds the death of speedsters and returns them to the Speed Force when they pass away.