I'm gonna let you in on a pie secret.
When you spend a lot of time laboring over a baking project, especially something like a pie or a cake, you want to make sure that it looks as good as it tastes. Of course, it doesn't have to be perfect to taste great—eating the "mistakes" is part of the joy of baking—but sometimes you want it to have that extra razzle-dazzle when you present your signature apple pie at Thanksgiving, or set out a plate of mini tarts at a party.
Once you have a finished baked good, there's a secret to giving it that pastry-case sheen, and it's fairly easy to reproduce. In pastry kitchens, chefs often use apricot glaze to brush over their finished pies and tarts in order to give the final product a soft shine. The glaze also slightly extends the shelf life of your baked good. If you make a ton of pies and tarts you probably already know this trick, and you can buy an industrial quantity of the stuff at a restaurant-supply store or on Amazon, where a 15-pound bucket of apricot glaze goes for about $50.
But for most home cooks, that quantity of apricot glaze is pretty excessive. You can recreate the same effect with apricot jam or preserves, ideally the kind that doesn't have many apricot pieces. Take a couple tablespoons of the preserves and warm it in a small saucepan with a little water or simple syrup, just until it becomes a consistency that you can easily spread. Use a pastry brush to gently coat the outside of your pastry. Your dessert will look professionally made but still have all the charms of your home kitchen.
This trick works the best for any dessert that has a pastry crust that's exposed, and for ones with fruit making up the top layer. It's not great for savory pastries. You might not want that subtle apricot flavor in your quiche, for example. But you can experiment at home and see what things you want to glaze and what you don't. Apricot is traditional because the flavor is subtle and the glaze is pretty clear, but there's no reason you couldn't try it out with other preserves, jellies, or jams to add some extra layers of flavors to whatever you're making.