Baking Blind (sometimes called "pre-baking") is the process of baking a pie crust or other pastry without the filling. Blind baking a pie crust is necessary when it will be filled with an unbaked filling (such as with pudding or cream pies) or when the filling has a shorter bake time than the crust. Blind baking a pie crust also helps prevent the pie crust from becoming soggy from its filling.
Generally, the pie crust is lined with tin foil or parchment paper, then filled with dried peas, lentils, beans or other pulses, or with ceramic or metal "baking beans" (also called pastry weights or pie weights) so that it will keep its shape when baking. Alternatively, the crust is poked repeatedly with the tines of a fork to produce small holes--this helps the steam escape and prevents the crust from bubbling up. After the pie crust is done, the temporary filling is replaced with the proper filling.
Blind-baking also helps to form a nice pastry case for the filling as it has already been partially formed from blind-baking.