The Kree Mact is a powerful energy device, originally in the possession of Ronan the Accuser. It was stolen from him by the Guardians of the Galaxy, and brought to Earth. There, it was taken into S.H.I.E.L.D. custody to be studied by Tony Stark.
When Ronan found out, he send his pet Fin Fang Foom to Earth to force the Guardians and Avengers to give it back. However, the Guardians and Avengers instead used the Kree Mact to powerup a beam weapon capable of defeating Fin Fang Foom. Angered by Fin Fang Foom's defeat, Ronan came to Earth himself to personally fight both teams and get his Kree Mact back. However, the Avengers and Guardians managed to defeat him and he was forced to retreat.
When the Guardians left Earth again, they left the Kree Mact with the Avengers. Tony then came up with the plan to harness the Kree Mact's power to create new battle armors for the Avengers so they would stand a chance against the Masters of Evil. Tony eventually developed the Build Up Armor, but it remains unclear if the Kree Mact was indeed used as a power source for these armors.
After the Avengers were finally released from their DISKs, but the Earth was threatened by a new evil, Dormammu, Iron Man planned to once more harness the Kree Mact as a power source for a weapon capable of defeating Dormammu. This plan resulted in the construction of 10 large energy cannons, powered by the Kree Mact. The plan worked and the full force of the 10 cannons was sufficient enough to destroy the gate to the Dark Dimension and severely weaken Dormammu, who was then D-Secured by Loki.
Loki then absorbed Dormammu's power for himself and defeated the Avengers before fighting Iron Man one-on-one, crippling the hero's armor. Nozomu Akatsuki suggested that the Kree Mact should be used to reboot Iron Man's armor. Akira and his friends thus took the Kree Mact to Iron Man. This plan worked as well, and the Kree Mact gave Iron Man the power boost he needed to finally defeat Loki once and for all.