Comments are a sign of bad code in two ways.
First of all, you add them to code that isn't clear without them. Good comments are comments that add clarity to code that isn't clear or isn't sufficiently clear without them. There's no point in belaboring the obvious or having a comment that fails to explain the nonobvious. Now imagine taking a good comment and removing it. You are now left with code that is bad, or at least not as good as it could be.