To crimp something -- say, pastry, pizza or pie dough -- is to decoratively fold the edges of it, slightly overlapping the folds to create a rounded border. 

Crimping is easy to do and makes the edges of sweet and savory pies, empanadas, dumplings and calzones look more finished. Crimping also seals two pieces of dough tightly to keep the filling ingredients from leaking out during baking.

Aluminum foil or parchment paper is also crimped when used to make a papillote. The sealed package seals in the steam that cooks the meat or fish.