What does it mean when someone says a discussion section is not "well-developed"? A reviewer of my paper told me that the discussion section is not well developed.

What does this mean? What should I work on to improve my discussion section?
If the reviewer says the Discussion section is not well developed, it suggests you either need to cover more of the elements that go there or go into greater depth on the things you do say there.

Carnegie Mellon has a nice handout about what different traditional elements of a paper should contain. 


  Discussion sections contain the following moves:
  
  
  They summarize the main findings of the study.  This allows readers to skip to the beginning of the 
  discussion section and understand the main “news” in the report.
  They connect these findings to other research
  They discuss flaws in the current study.
  They use these flaws as reasons to suggest additional, future research.
  (If needed) They state the implications of their findings for future policy or practice.
  


Further, if the Results section does not comment on the results, such as attempting to explain them and evaluating whether the results support the hypothesis, then that should be part of the Discussion.

Finally, as henning recommends, let a trusted colleague read the paper and ask for their suggestions for improving the discussion section.