What is happening in this line of code?

I am trying to write a script that uses the nlapiSubmitField function but I can't seem to get it to work.
I had some help writing the following line and I can't seem to understand what is happening here.
this function is on the item receipt record.

if (nlapiGetLineItemValue('item', 'upc', i))
           {
               nlapiSubmitField('inventoryitem', nlapiGetLineItemValue('item', 'item', i), 'upccode', nlapiGetLineItemValue('item', 'upc', i));
            }

what I THINK is happening is:
1) (nlapiGetLineItemValue('item', 'upc', i)) is scanning through the line items on the item receipt and looking to see if the UPC field is populated
2) IF the UPC field is populated then