Food grade pickling lime (also called calcium hydroxide, slack lime, slaked lime, lime, calcium hydrate, etc.) is a type of white chemical powder that was a once-popular addition in pickle recipes to help enhance the "crispness" of the finished pickled.  The way it worked was to add calcium into the natural pectin in the food being pickled. In the case of candied pumpkin, it helps the pumpkin pieces to maintain their shape and texture when simmered for hours during the candying process.  However, this ingredient is also relatively controversial and not often recommended in modern preserved recipes. This is because it can impact the pH level in the food (as it's alkaline) and could potentially lead to botulism in home-canned foods unless the lime is thoroughly rinsed from the food (usually several times). You can use pH strips to check the acidity of the pumpkin preserves before canning.