A mobile ticket is a ticket that is imported into the NS International App on a smartphone (Android or iOS). It is in the shape of a square barcode which contains the details of your journey. The barcode is scanned when it is checked on the train. You can also use it to open the gates at Dutch railway stations.
You can import your mobile ticket by installing the NS International app on your mobile phone, opening the booking confirmation on your mobile phone and by tapping the Tickets in app button.