In French, to cook something en papillote means to cook it in a folded pouch of parchment paper that is tightly sealed before baking. 

Cooking en papillote is healthy and flavorful, as it seals in the juices of and steam-cooks whatever's inside. We love to cook fish with herbs, aromatics, wine and a smidgen of optional butter or oil. Individual papillotes can be prepared ahead of time and cooked at the last minute. (We recommend doing this if you're making them for more than two people.) Serve them unopened and let each person inhale the heady rush of aroma that escapes when the papillote is sliced open.

Papillotes can also be made using aluminum foil; these work beautifully on the grill (or over low coals in the fireplace). Foil pouches can simply be creased shut. The parchment variety can be folded as well, but they also benefit from the extra security of staples or a paper clip.

Bananas en papillote made with Grand Marnier and fresh oranges is an easy desert. "Papillote" is also the name of an old-fashioned Lyonnaise bonbon as well as the frilly paper used to dress up the bones on a rack of lamb.

Reference: How to fold a papillote (Cookthink)