Pie weights are weights that you use when pre-baking a pie crust to keep the dough from puffing up or pulling away from the tin. They are most often small ceramic balls, but may also be metal balls, chains (coiled in the base of the pie pan), beans, rice, or anything else heavy that you can cook at high temperatures.
The most common method of weighting a pie crust is to shape it into the pan, line it with aluminum foil, parchment paper, or cheesecloth, and then spread the weights evenly across the base of the pie. It is not required to fill the pie crust, as most distortion will occur on the floor of the crust. Once the crust is cooked, remove the weights, add the filling, cook further if required, and serve.