Preserving Sugar is white sugar that comes in crystals larger than those in granulated sugar.
The idea is that because the crystals are larger, they will take longer to dissolve completely, helping to prevent sugar congregating at the bottom of your pot, or rising up as froth to the surface.
Because it minimises scum, it helps to make jellies clearer. It also reduces somewhat the need for non-stop stirring.
Preserving Sugar is different from Gelling Sugar, because Gelling Sugar contains pectin and Preserving Sugar is just sugar.