What does the middle candle of the menorah represent?

The shamash does not represent anything or stand for anything. It is the "servant" candle used to light the others and to provide light. The other 8 candles stand for the 8 days of Hanukkah. The shamash is usually the center candle and stands out from the others by being elevated, but it can be at the end or the row or in front of the row, so long as it stands out from the other 8.