Technical debt is code that makes everybody waste time (eg. because of its complexity), or that creates problems in production (eg. performance issues) BUT that

Technical debt is code that makes everybody waste time (eg. because of its complexity), or that creates problems in production (eg. performance issues) BUT that you cannot easily replace because it is too tightly knit to the rest of the architecture, or it would be very long/risky. So you have to live with and pay its dividends every year. The most frustrating thing for me is that you cannot throw it away and start from scratch (if you want to keep your users). You have to bite the bullet and find ways to make that ugly/smelly code go away in very small increments. This process can take years.