Iron Door

The iron door is a block that can be opened and closed with redstone circuits. They act as a barrier to keep hostile mobs such as creepers, zombies, and skeletons out of areas as they cannot open doors.

An iron door (via screenshot)

How to get an iron door?

Iron doors can either be crafted or found in naturally generated structures such as woodland mansions and strongholds. In stronghold, they are generated along with a button next to them. You can pick up these doors by breaking them with a pickaxe. Another way to break them is by destroying the block below them. This will cause the door to break and drop itself as an item.

Crafting iron doors (via screenshot)

What is the use of iron doors?

An iron door can only be opened with a redstone signal. Doors are used as barriers to keep hostile mobs out of rooms, buildings, etc. They can also be used to block entities such as boats and items. When placed underwater they make an air pocket that can be used to breathe and stay alive. The only way mobs can open iron doors is by either stepping on a pressure plate or by triggering a tripwire.

An iron door found in a stronghold (via screenshot)

Conclusion

Iron doors are a variant of regular wooden doors that cannot be opened by mobs. They cannot be opened by right-clicking them but redstone power from buttons, levers, pressure plates, etc. can be used to open them.