Software systems are prone to the build up of cruft: deficiencies in internal quality that make it harder than it would ideally be to modify and extend the system further. Technical Debt is a metaphor, coined by Ward Cunningham, that frames how to think about dealing with this cruft, thinking of it like a financial debt. The extra effort that it takes to add new features is the interest paid on the debt.
Imagine I have a confusing module structure in my code base. I need to add a new feature. If the module structure was clear, then it would take me four days to add the feature, but with this cruft, it takes me six days. The two day difference is the interest on the debt.