The iron trapdoor is a solid block that works like a horizontal door. It cannot be opened by mobs or players by right-clicking. Like iron doors, iron trapdoors also require redstone power to open. When placed outside rain, snowfall, and water aren't able to pass through it, but light can shine through them.
An Iron trapdoor (via screenshot)
Iron trapdoors can be either found as part of naturally generated ancient cities or you can craft them yourself. To pick up naturally generated ones you will need to use a pickaxe to mine it after which it will drop itself as an item. It is easier to just craft them rather than go looking in ancient cities. To craft an iron trapdoor you will need to arrange 4 iron ingots in a 2x2 layout on the 3x3 crafting grid.
Crafting an iron trapdoor (via screenshot)
A trapdoor can be used as a switchable barrier to entity movement. Although primarily used to block movement by mobs and players, a trapdoor can also be used to control the movement of boats (for example, a top trapdoor placed in a two-wide water flow stops a boat when closed (extended out into the water flow), but allow it to move again when open), items and minecarts (a trapdoor can stop a falling item or minecart, then allow it to drop again when the trapdoor opens), etc. Iron trapdoors can be opened only with redstone power.
Iron trapdoors are a variant of regular wooden trapdoors that cannot be opened by mobs. They cannot be opened by right-clicking them, redstone power from buttons, levers, pressure plates, etc. can be used to open them.
A. To craft an iron trapdoor you will have to use 4 iron ingots in a 2x2 pattern.
A. Iron Trapdoors were added to Minecraft in Update 0.13.
A. Yes, water will not flow through or around the iron trapdoor.