what does “docking” mean?

To dock a dough means to basically aerate it, creating small holes that will allow trapped air to escape under the dough during baking so that it doesn’t billow up or puff when you want it to bake flat. A docker is the tool that makes it happen; A roller with lots of pins protruding from it that create holes as you roll it along dough. If you get one, choose metal pins instead of plastic because they don’t warp.