Metal bridge is one of only a handful of items that have collision for players and mobs, but not for liquids or rainfall. You can find this item in the Architecture section of inventory. It is 3 blocks tall and 4 blocks wide and, like any item greater than 1x1, can be layered.
How to Obtain
This item generates naturally in certain dungeons and can also be crafted or traded.
How to Use
Often used as a way to let steam pipes into a structure while keeping mobs out, or as a way to build above an area while still letting rain through so compost can grow. Note that the corresponding Metal Bridge Block DOES have full collision.