Coverage testing is a great way to find out what parts of your code are not tested by your test suite. You turn on coverage.py, then run your tests. At the end, coverage can show you which lines were never executed, either by line number or visually in an annotated source file.
When your test coverage is less than 100%, coverage testing works well: it points you to the lines in your code that are never run, showing the way to new tests to write. The ultimate goal, of course, is to get your test coverage to 100%.
But then you have problems, because 100% test coverage doesn't really mean much. There are dozens of ways your code or your tests could still broken, but now you aren't getting any directions. The measurement coverage.py provides is more accurately called statement coverage, because it tells you which statements were executed. Statement coverage testing has taken you to the end of its road, and the bad news is, you aren't at your destination, but you've run out of road.
By way of illustration, here are a few examples of 100% statement coverage of buggy code.