Oh boi, such a question you asked. This ring is incredibly vast in lore, it'd take quite some time to get all the details. There's a page at Halopedia
that covers the ring's whole story and such. But it's essentially the turning point of the Halo Universe.
Installation 07 or Zeta Halo (as I will be referring to) was originally part of the 1st array of Halo rings. Born from the foundries of the Greater Ark, Zeta Halo used to be 30,000 KM in diameter, but was reduced to 10,000 KM after heavy structural damage and re-linked to the second array of Halo rings, the one we encounter in the games.
This ring was home to many different species and races. I remember Primordium talking about the ring housing different races of Humans, like Chakas' and Riser's (the latter was "a short florian" something along those lines. Read this from Cryptum), but was also home to the Palace of Pain, a horrible place in which Faber, the Master Builder, conducted numerous horrible experiments, especially on Humans, in order to find the cure for the Flood. The ring is also home to the core of the Contender-class AI Mendicant Bias. Faber decided to put all control of the rings and the Forerunner military under this AI, but little did he know that this would later be a grave mistake.
After the Forerunner-Precursor "War", the Primordial, believed to be the last Precursor alive, was held captive at Zeta Halo and Mendicant Bias interrogated it. For 43 years, they conversed as the Flood spread through the Galaxy and Mendicant was infected with the Logic Plague and turned against his masters. This ring would later be used by Mendicant against the capital city of the Forerunners, an event which later became known as the Fate of Maethrillian. I believe this is where Mendicant lost control of the ring, which would have suffered all the major damage (additionally, some rings were completely destroyed in this battle).
So, after the Forerunners realized their errors and that there was no way to win the war, they built the second array, repaired and linked Zeta to that array and then was activated by Bornstellar, along with the other 6 Halos. This successfully hindered the parasite across the whole Galaxy and would leave it starving for the thousands of years that came. What happened with Bornstellar and other probable Forerunner survivors is unknown, but they likely left the Galaxy, seeing themselves as unfit to continue serving it.
Fast-forwarding over 100,000 years, Zeta Halo would later be discovered by Humanity after Delta Halo's sudden deactivation by Miranda Keyes in 2552, but wouldn't be visited until 2555, where it was discovered that the ring, along with all the others, were about to fire in just 5 weeks by order of 000 Tragic Solitude. As you can guess, they were successful to stop this.
Now we find ourselves wondering what purpose could the ring have after it's reveal at E3 2018. I'm very curious.