Margin refers to the amount of capital required by PU Prime to open and maintain a position. Margin usually relates to leverage. When you trade on margin, you are essentially leveraging your position. By depositing a fraction of the total value of the trade as margin, you can control a much larger position.
For example, if a trader has $1,000 in their account and uses 30:1 leverage, generally they can control a $30,000 position.