It says that, roughly on average, you should be able to cover 80 percent of the offending cases by doing just 20% of the work. That is, by writing tests for just 20% of the code, you can probably catch 80% of the bugs and regressions. That's because most of the fragile code, commonly changed code and worst offending code makes up just 20% of the codebase. In fact, it may be even less.