Candi sugar is a Belgian sugar commonly used in brewing, especially in stronger, Belgian beers such as dubbel and tripel.
Chemically, it is an invert sugar: one that has been converted from sucrose to a mixture of fructose and glucose.
It is used to boost the alcohol content without adding extra body to the beer, and without forcing the yeast to produce invertase.
Also used as a priming sugar, to aid in bottle-conditioning and carbonation, with the same benefits as listed above.