You think the price of silver is going to fall over during the day. You sell 5,000 oz, or 1 lot, of XAG/USD at the price of USD16.38. One lot is 5,000 oz x USD16.38 = USD81,900 (or USD50 for every 1-cent movement in the price of silver).
Later that day silver is trading at USD16.25, and you decide to buy back to close. Your profit on the trade is USD650.00 (USD16.38 closing price - USD16.25 opening price x 5,000 oz).
Using leverage of 200:1, your initial margin would have been 5,000 oz x USD16.38 x 1 lot / 200 = USD409.50.