Screen "Goods" contains list(catalog) of all your goods including information about quantity in stock.
You add items to documents selecting them from this catalog. Moreover, if item has a barcode it can be added to document very quickly simply by scanning this barcode with camera. Quantity in stock is not editable, it is calculated using data in your documents.

You can add items to catalog enetering data manually or importing list from Excel spreadsheet.
For importing data from Excel file it should contain 2 columns - Name and Barcode. Which columns should be used for import you can specify in "Settings". By default apllication uses columns A and B.
During manual entry you can read product barcode with camera by pressing button next to barcode field.
To delete item from the list long press or just swipe the item.

For search and sorting you can use top toolbar buttons.

You can start working without filling in Goods catalog - you can create Incoming document, enter information about new products there manually and these positions will be added to catalog automatically. Next time you will be able to use barcode scanner and add these items to document by barcode.