Reconciliation Debt is the damage caused to spacetime by a ship moving through Slipstream Space. It was an important feature of Forerunner Slipstream technology and was important to their commerce.
During the time of the Ecumene, the galaxy in total had racked up incredible amounts of "debt" due to the scale of the Forerunners's actions. The movement of the Halo Arrays required so much Reconcilliation that it disrupted and prevented some ship travel for centuries.
Travelling through Slipspace violates causality, as it is a form of faster-than-light travel. When moving faster than light, the nature of spacetime and Relativity causes certain events to preceed their causes, and makes possible paths where a ship arrives before it leaves. These violations are incompatible with space. The design of forerunner drives, the fastest ships know to exist, accounts for this damage by reconcilliation, which can be seen as a glow similar to cherenkov radiation emitted from a ship moments after it exits slipspace. A ship generally disperses this "debt" when exiting slipspace. However, if too much "debt" is created, the ship will be destroyed upon reentry. During the Forerunner-Flood War, movements of enormous volumes of matter (Primarily the Halo Array) and ships travelling as far as the Small Mallengenic Cloud accrue so much Reconcilliation Debt that space-travel is disrupted through the galaxy for centuries, with lower civilizations often finding their slipspace drives dangerously inoperable.