The Structure Block can be used for copying structures in your world and then pasting them somewhere else. It's a new type of block which according according to the Minecraft wiki is an existing feature in Minecraft Pocket Edition but can't yet (officially) be obtained. However, if you install this add-on you will be able to access the block and use it in-game.
Creator: ModelPart, Twitter Account
How does it work?
To demonstrate this feature I am going to show you how to copy a structure and then paste it somewhere else.
I've added one video tutorial by RageElixir and another by EckoSoldier to the bottom of the post.
Place down a structure block at the lower bottom corner of the structure which you want to copy. Tap on the block and select Corner Mode and enter a name for it, e.g. "House1". Then press Accept.
Find the opposite top corner of the structure and place down a second structure block. Press on the block, select Corner Mode and make sure to the same name which you used before, e.g. "House1". Then press Accept.
Next, place down a third structure block and then tap on it once. Select Save Mode, name it "House1", press the Detect button and then hit Save.
Go to the area where you want the structured pasted and place down another structure block. Tap on the block and select Load Mode. Enter the name for the structure "House1" and then press load.
Before it actually spawns it will show an outline of the structure. This is useful in case you want to make any changes to the position of the structure. If it's all good then tap on the structure block again and press Load.
I'm sure it can be used for many other things but this will give you an idea of what you can do with it in-game. Check out the Minecraft wiki for more information about the block. Keep in mind that this is a BlockLauncher addon and that it might not include all of the features which the official feature includes.
Important: This is a BlockLauncher addon which will only work on Android devices!