Jat-Krula

The Maginot Line, otherwise known as the Maginot Sphere, was a Forerunner defensive line that protected the Forerunner inner colonies. It divided what the Forerunners were able to protect from what they had to leave to the Flood. Its precise location is unknown, but from records drawn from the Halo 3 Terminals, we can assume that Earth was not inside it and neither were most of the worlds the Librarian visited in her duties.

It was heavily damaged when the Flood made a massive attack on it, led and commanded by the rampant Forerunner AI Mendicant Bias. It was the primary line of defense holding the Flood back. It is assumed that it was broken before the first firing of the Halo Array. It was a sphere around a Forerunner-controlled area of space.

It can be assumed that for a long time the Didact worked in the Maginot sphere, frequently sending transmissions from it to the Librarian asking them to retreat behind it to where the Didact said their Fleets could protect the Librarian. Although it was a defense against the Flood, it cannot be assumed that the Flood conquered all worlds that were outside of the Sphere, as Earth which we know was outside of it was presumably not infected because the Librarian was on it, looking at how beautiful it was before the Array fired.

The Maginot Sphere may have been a physical object, (an energy barrier of some sort) as a Keyship was needed to enter it. It may also be a hypothetical line, not a physical object, used to mark what could and couldn't be saved, and the fleets guarding it would not permit you to pass unless you were in a Keyship.

It's assumed that the Maginot Sphere isn't very large considering you can see the milky was from the surface of the Ark. which means there are very few Forerunner Inner colonies or their spaced relatively close together, since it is clearly stated that earth was nowhere near the Maginot line.

Trivia

 * The Maginot Line was a World War II French line of fortifications, meant to repel an anticipated assault from Germany. Though it dissuaded a direct attack, German tank units maneuvered around the fortifications, bypassing most of them altogether, making the fortifications mostly useless and leading to the fall of France. When the Allies retook France, the Germans manned them, but again the majority of the lines were bypassed.