The term blind-baking refers to the process of baking of a pie crust without the pie filling.
Generally, the pie crust is lined with tin foil or parchment paper, then filled with dried peas, lentils, beans or other pulses, so that it will keep its shape when baking. After the pie crust is done, the pulses are replaced with the proper filling. Blind-baking is necessary if the pie filling can not be baked as long as the crust requires, or if the filling of the pie would make the crust too soggy if added immediately.
