When you add a barcode onto a layer, a visual representation of the barcode is shown to provide an idea of the size of the screen of the target device. It is important to make the barcode large enough for your equipment to read it.
The data fields which will be “encoded” into the barcode are displayed on the right. In the example below 
“first name”-“Last name” will be encoded into this bar code e.g. bill-smith