It is very important that the boarding pass on your mobile has clearly visible bar cod. You will bring your mobile to the scanner before boarding on the plane and you will be reported that you have boarded.
Most of low cost airlines allow you to show boarding pass on the mobile during boarding procedure. However be informed if you need to print and take it with you.
If you fly with RyanAir and do not have a European Union passport you should consider some information. When you complete your online check-in, you will receive an e-mail boarding ticket that you have to print.
If you see the Visa Check mark (see picture), you should search after the RyanAir info desk at the airport.
You will give the officer a passport and a printed boarding card to put a stamp and sign. This is to confirm that you do not need the visa for the country of travel or, if necessary, that you are in possession of it and verified by stamping and signature.
In these cases, it is not enough to have the boarding ticket on your mobile, but you need to have it printed. Take care, airlines charge up to 15 € for a boarding ticket printing service.