Breaches

A breach is hole or rift in the dimensional barrier, bridging worlds together. Fifty-two breaches between Earth One and Earth Two were created when time was tampered with when Barry went back in time to save his mom. Normally only speedsters can pass through breaches, but somehow Harry was able to cross over to Earth One using a breach. It was said that Zoom could see and hear through the breaches, but it's unclear whether he could himself or was using Reverb to do it, who could at least open/close portals to Earth One, and potentially more, and so can Vibe.

History
Zoom appears out of a breach with Sand Demon. He asks Zoom where he is and what he has to do to get home, and Zoom simply tells that all he has to do, is kill the Flash.

Barry tries to speed through the breach under S.T.A.R. Labs, but isn't fast enough, and later, Harry comes through the breach.

Barry is set on stopping Zoom, so the team choreograph a fight between him and Doctor Light to lure Zoom into an ambush, but he doesn't fall for it and comes through it later and captures Linda to get Barry's attention.

Before going to Earth-2 to save Harry's daughter, Jesse, Barry closes fifty-one breaches, leaving only open, through which Barry, Cisco and Harry use to cross over to Earth-2. The breach behind them destabilizes, but Jay and Caitlin fix it, and Barry, Cisco, Harry and Jesse eventually come back through, and seal it, but Zoom manages to grab and kill Jay, who is actually his time remnant.

With encouragement from Barry, Cisco taps into the energies that binds the multiverse together, as is his power, and reopens a breach to Earth-2 so that Barry and Zoom can face-off. But, Zoom gets away after a mini fight with the Flash, and takes Wally hostage as a bargaining chip. Barry gives Zoom his speed for Wally's life, further enhancing Zoom's speed way beyond what it was before and curing him.

As a way to subdue the Earth-2 metas, the team sends out a high-pitch frequency specific to that of the frequency that Earth-2 vibrates at, affecting people from Earth-2, even Zoom, so Zoom creates a breach with his own power and hops through it.