The function of a discussion section is to:
interpret the results presented in the results section; and
discuss them in relation to your research question and to the results of previous research in the field.
Of course, to present any discussion about results from previous research, you must already have introduced this research in your literature review. Discussion chapters also often include sub-sections on issues arising from the study, or that detail the implications of the research.
See Biology Discussion & Conclusion (PDF file) section.