This sugar is an unrefined cane sugar that is popular in English Ales. It contains small amounts of molasses and other natural compounds meaning it produces beer with more character than refined table sugar. It is about 97% sucrose which means most of it will ferment out and it will not impart much body to your beer. It is generally used in darker beers since the color of the sugar itself will affect the color of your brew.
You could even try it out with your darker beers as a priming sugar!