Blind baking is another term for prebaking, and it refers to a pie or tart crust that you partially or completely bake before it is filled. This is done in many cases to help keep the crust from becoming soggy from a wet fruit filling, or so that you have a cooked crust if you are filling the pie with something already cooked, such as a custard.
Generally to prebake a crust, you roll it out and put it in the pan. To keep the bottom from puffing and the sides from falling, you should line the crust with parchment paper or a large coffee filter, and fill it with beans or rice. There are special pie weights on the market, and some people might encourage you to line the crust with foil, but Rose Levy Beranbaum, author of The Pie & Pastry Bible, says you should use neither. The foil keeps the crust from beathing, she says, and the weights are simply too heavy, often producing a "cardboard-y" crust.
Make sure to gently push the beans or rice up against the sides of the parchment or coffee filter, to keep the sides of the crust from collapsing in the heat of the oven. Place the crust in a hot oven (say, 425°F; 220°C), which will help set the flour in the sides before the fat starts to soften, and bake for 20 minutes. Carefully remove the weights and liner from the crust, prick the bottom with the tines of a fork to allow steam to escape, and return the crust to the oven.
If you are prebaking the crust, it may only need another 5 minutes in the oven, until it is a very light brown. If you want to fully bake the crust, it may need 10 to 20 minutes more baking until it is done. You may also have to prick the bottom again if it gets uppity.
Now, having done all this prebaking, blind baking, or baking blind, you also must take care if you're subsequently going to add a filling and bake some more that you don't overcook the edges of the crust that you so magnificently crimped or fluted or otherwise decorated. You can buy a pie crust shield online, or if you're a master of tin-foil origami, you can make one of your own.