To speed up picking an order, a barcode is printed on the invoice or delivery note. It's displayed below the top right block, which usually contains the shop's address as well as the customer number, date, etc.
If the icon shown below is displayed instead of the barcode, please check the allow_url_fopen option in your server's PHP configuration. This value must be true for the barcode to be displayed correctly.
If the barcode is missing entirely, please check whether the HTML element with the ID head_right, in which the barcode is added, is included in the invoice template index.tpl you're using. If not, add such an element to your invoice template.