Docking Field is an object unobtainable by players through normal means. This item's information is present in the game files, but not intended by developers to be in players' possession.
It provides a visual barrier which signifies gravity and atmosphere in space. After passing through it from space players will be able to breathe and have gravity again. (Note that this effect is purely cosmetic, because air and gravity are controlled in the game by the dungeonid assigned to each block in a world, not by objects.)