Shield Controller is an item that protects player's armor from any kind of damage. Especially, it's intended to protect armor from corrosion due to the acid rain atmosphere on Venus.
When the Shield Controller is equipped, the player will be drawn surrounded by a visual effect similar to a force-field.
While in Venus, it is recommend to use "disposable" armors until getting the Shield Controller since there is no other way to protect armors from corrosion (details on GitHub).
Note: Sometimes the treasure chest will generate a Volcanic Pickaxe instead.
The Shield Controller is a non-craftable treasure item.
To equip the Shield Controller, open the Galacticraft inventory tab, and place the item in the topmost slot.
You can see it protecting the player's armor (the animated overlay covering the whole body) in this Venus boss test video (at 0:11 and 4:06).