Design debt accumulates when decisions get made under pressure without enough thought. It shows up as inconsistencies, workarounds, and interfaces that require explanation rather than affording their own use. At a certain level it slows the team down, every new feature has to navigate the existing mess. Tracking it as a metric makes the invisible visible rather than leaving it as a vague sense that things are getting harder.
Implementing a design system is one way to pay it down, but only once the debt has been made legible.