The Business Items are nothing more of the inputs and outputs that get generated in a process. Is the thing that gets created, modified or destroyed in the process (ok, remember that in nature nothing gets created or destroy, just transformed)
Here it is how a Business Item looks in a process:
But the business item, in this "Application" has a lot of stuff inside it.
Just looks the attributes you can add to it, you can even put a "Business Item" inside other "Business item"
Or the documentation stuff, text, files and links.