I'd venture that it refers to bad decisions that lead to low quality of code, the problems of which are inherited by all code built on it. And so on. With every layer you pay for your past mistakes, and you quickly spend more time working around the problems than you would have spent fixing them in the first place. (or fixing the stuff below before layering even more code on)

For example, the amount of hoops I had to jump through to be allowed to fix some old code at my last job was astounding, even though the bugs had literally caused man-years of lost work, and eventually took me about 2 weeks to sort out.