Pearl Sugar is a kind of refined white sugar, very coarse, hard, and opaque white. It typically does not melt at regular baking temperatures. It can be used as a sweetener, of course, but it is mainly used to decorate desserts. So, you can find it in many recipes, from cookies to cupcakes.
Quite easy to find, you can buy it in any supermarket, in the cake-decorating section, usually in 125-gr packeges.
If you want to try and make it at home, you will have to amalgamate sugar with water (no more than the 10% of the sugar weight). Then you need to push the mixture through the holes of a perforated spoon, and let it dry for several hours.