The title Fashion Merchandiser defines a category of people who buy, sell, manufacture, or promote fashions. In general you do not work as a Fashion Merchandiser, but rather you pursue a degree in "Fashion Merchandising":/degrees/bachelors-fashion-merchandising and then go on to a career as a Fashion Buyer, Apparel Product Developer, or some other specialized field that falls in the realm defined above.
Because this is a broad title, your daily tasks as a Fashion Merchandiser will vary with each position; you might be buying new fashions, organizing store displays, or analyzing financial forecasts for the industry. In large companies like department stores you will be very specialized and only work in one of these areas, whereas in smaller companies you might hold responsibilities for several.
Regardless of your specific position however, your goal as a Fashion Merchandiser is to sell maximum product by make the shopping experience as interesting and engaging as possible.