Position Papers are the culmination of your research for MUN, and are required for most every collegiate conference. In essence, they are the homework that you will bring to committee. The short (1-2 pages) report will outline your country's position on the issues to be discussed in your committee or cabinet, and will be used as reference for both you (so you can stick to your nation's policy correctly) and your dais (so, when the chairs decide awards at the end of the conference, they will decide whether or not you played your part well by looking at your position paper and your participation in committee).
      The UNAUSA provides excellent resources for writing position papers on their website, including a sample position paper to get your started.