When baking pie and tart shells, you are often instructed to dock (poke holes into) the dough before baking. You can get a fancy docker that looks like a lint roller with spikes, or you can just use a fork. Docking the shell allows air to escape when you bake it. Tart and pie dough have a fair amount of liquid in them from the butter, eggs, and any other liquid you've mixed in. When the dough goes into the oven, the liquids turn to steam with the heat. This steam can cause big bubbles to form in the shell unless you give it a way to escape. Hence you dock pastry shells. If you are filling the tart with a liquid filling after you blind bake it, take care not to dock too aggressively. A few holes here and there will be enough to release the steam but not so much that the liquid filling leaks out.