"If everything you saw is true... the end of the world is far closer than we'd like."
The Kree Timeline is a conjectural name given to a potential future timeline. Progressing into an unknown point in the future, it is a version of Earth where the Kree Empire invaded and wiped out most of humanity.
The Kree Timeline is believed to follow the same events of the current timeline. However, at an unspecified point in the future, the Kree Empire attacked the planet and engaged in a battle with the inhabitances. This led to a global battle that irreparably damaged the planet's environment, disrupted tectonic plates and wiped out most life on the planet. Coupled with nuclear weapons being used by both sides, Earth was effectively transformed into a Wasteland. Eventually, the Kree Empire successfully invaded Earth by wiping out the last defenses with their Sentry robots. It is believed that all resistance bodies were either wiped out or severely outgunned, including the Avengers.
In the fallout of the war, Bruce Banner was driven insane by the sheer destruction caused by the war and the excessive radiation levels created in it. This lead to him permanently transforming into the Hulk, though their personalities merged to create an intelligent but vicious personality called "Maestro". Declaring himself the ruler, Maestro battled and defeated all heroes and villains, collecting trophies to remember his conquests. Having retired from the Avengers before the invasion and been inactive in these events, Hawkeye began working to repair the world and help those in need.
Having also survived the invasion, Monica Rappaccini continued leading the remnants of Advanced Idea Mechanics and attempted to continue the fight against the Kree Empire. At some point, she was able to construct Time Bridges to contact both past versions of herself and the company. Monica intended to use the available resources to finish a weapon she was developing to cripple the Kree's invasion force and receive help from dead or missing individuals. Through the past version of AIM, Nick Fury and Hawkeye both sent to help her in the timeline, but recognized she'd already lost and that it would be better to prepare their version of the Avengers before the invasion.