The Breakout is the event when an unknown individual have messed up the security systems of four major supervillain prisons (The Big House, The Cube,The Vault and The Raft), all 74 supervillains have escaped and it caused to the formation of the Avengers to protect the world from their menace.
As all of the 74 supervillains escaped, individual heroes and security forces tried to apprehend them but to no success in keeping them from escaping.
As Graviton escaped from his anti-gravity cell and rampaged around Manhattan, all five heroes were united to stop him at all costs. Taking him on together, they defeated Graviton.
After the Breakout, they were asked by Nick Fury to join S.H.I.E.L.D. as part of a new team he's forming to recapture the villains but the five heroes refused to join since S.H.I.E.L.D. has done enough damage. Then they decided to form their independent team of their own, the Avengers.
In the aftermath of The Breakout, 74 supervillains are now loose all around the world and went into hiding while planning their evil schemes. All four prisons were abandoned or destroyed after the massive breakout, which paved way for the creation of Prison 42 in the Negative Zone by Iron Man, Ant-Man, and Mr. Fantastic for the recaptured supervillains.
It is revealed that Loki is responsible for causing of The Breakout, which released all imprisoned supervillains just to keep Thor busy while he took over the Nine Realms with ease.