For most airlines, after online check-in, you will receive an SMS and e-mail that includes your boarding pass information with a barcode. If you only have cabin-size baggage, you can pass the security gates just by showing this barcode and your ID to the relevant officer and board directly. However, if you have baggage that needs to be delivered to the plane, you should go to your airline’s counter and deliver your baggage there. Then you can board the plane with your barcode boarding pass and your ID.