This app calculates how much storage capacity you never will be able to use on your storage device. Or the opposite - how much extra storage youll have to buy to get the space you want.
If you dont understand what Im talking about above, then maybe this will make it clearer:
Have you ever bought for example a USB-memory with 8GB storage capacity and when you plug it in you see that its only like 7,45GB? Or a hard drive thats 1TB and when you plug it in theres only like 910GB (0,91TB)?
This happens because computers use the binary number system (base 2) and not the decimal number system (base 10). When a manufacture says that a hard drive have 1TB of space, its true. But because computers uses the binary-system theyll interpret 1TB as 0,91TiB. The "i" stands for binary - 0,91TerraBinaryBytes. So for example: 1GB is 0,93GiB (if you buy a USB-memory whichs on 1GB you can only use 0,93GB).
When you launch the application itll speaks for it self. You can either see how much storage youve "lost" or how much extra you must buy to get the space you want.