I don't think this point can be emphasized enough. When you comment code, your primary focus should be on the "why".

In all but very specific circumstances, the "what" should be fairly obvious by reading the code, with small clues being sufficient to guide the reader.

However, without any comments then there is no way for the reader to understand why a particular approach was chosen. Worse, they may think it needs to be refactored, and then waste time pursuing that only to arrive at the all-too-familiar "oh, that's why they did it that way".