Multiverse

The multiverse is a concept that refers to the existence of infinite universes that comprise everything that is. Each universe within the multiverse is referred to as a different "Earth", and each vibrates at a different frequency so that they cannot normally interact with or be seen by each other. If one is able to travel fast enough it is possible to breach the laws of physics and travel between these Earths.

History
In early 2015, Barry Allen and the rest of Team Flash accidentally opened a singularity above Central City; though they succeeded in closing it, a series of 52 breaches between Earth's One and Two were opened throughout the city as a result of their actions.

Between mid to late 2015 and early 2016, an Earth Two speedster known as Zoom would later use the breaches to launch an assault on Earth One's Central City in an attempt to steal The Flash's speed. Numerous meta-humans were sent through the breaches, each tasked with killing The Flash, but none succeeded. Team Flash, in an effort to stop Zoom's onslaught, systematically sealed each of the breaches, leaving only one open on a lower level inside S.T.A.R. Labs. Barry, Cisco Ramon, and Harrison Wells used the breach to travel to Earth Two in order to rescue Jesse Quick, sealing the breach upon their return.

Attempting to increase his speed, Barry Allen used a tachyon device, which in turn caused him to run too fast and accidentally create a breach between Earth One, and an Unnamed Earth, which is the home to Supergirl.

It was revealed by Hunter Zolomon that Earth One happened to be in the center of the entire Multiverse.

Known parallel worlds

 * Earth One
 * Earth Two
 * Earth Three
 * An unnamed Earth (home to CBS' Supergirl)
 * An unnamed Earth (home to CBS' The Flash)

Season 2

 * "The Race of His Life" (mentioned)
 * "The Race of His Life" (mentioned)
 * "The Race of His Life" (mentioned)
 * "The Race of His Life" (mentioned)

Trivia

 * The fact that the number of breaches that opened in Central City is 52, can be a possible reference to multiverse crisis known as 52 and New 52 that happened in the comics which created a new continuity for DC Comics.