Technical debt is the accumulated cost of past shortcuts in a codebase: quick fixes, outdated dependencies, undocumented logic. Like financial debt, it is not inherently bad — shipping fast can be worth it — but it charges interest: every new feature takes longer, breaks more, and depends on the one person who remembers why.
The management rule: make it visible, price it, and pay it down where it blocks the roadmap — not everywhere. Ignoring it entirely is how rebuilds get sold as the only option.