The Armor Stand is a block that you can place in the world and use to store and display armor.
It can also hold items. By holding shift, and right clicking with an open hand, a GUI will open. You can add armor and items, but the offhand will not show. The GUI has a glitch that that misplaces the items one row down in your inventory. Also, you can just right click with an open hand to swap the armor you are wearing with that on the armor stand.
As the Armor stand was added in version 1.8 of Minecraft, the mod may not contain this block much longer.