The barcode scanner is for speeding up inventory item Name data entry. For example, if you have a 15 oz can of Kroger Black Beans, instead of typing all that in via your mobile phone's on-screen keyboard, you can scan its barcode and it will enter "Kroger Black Beans, 15 oz" into the current item's Name field, where you can further edit it.

Add a new inventory item by tapping the + tool from the Inventory tab:

You can also access the scanner for an existing item, by tapping its Name from the Item Details screen:

Then tap the search icon:

And finally, tap the barcode icon to bring up the scanner:

Once the scanner is up, position the barcode as shown below:

You may have to move the barcode a little closer or farther away from the camera, but the scan should eventually complete:

The app will then search the UPC barcode databases to find the name, and optimally the size, of the item, and populate the item's Name field: