If you have booked a return trip, there will be two tickets attached to your e-ticket email, they will be labelled as either OUT or RTN to help you identify which ticket applies to which direction of travel.
If you have booked for multiple passengers then you will see that the tickets are distinguished as Adult or Child and the type of railcard used if applicable.
Some stations barriers will have a scanner to allow you through, simply present the barcode to the scanner and you will be allowed through the gate.
If you encounter a gate that does not have a scanner, you just need to show your eticket to a member of staff on hand at the barrier.
You may also be asked to present your ticket on board the train. Again simply show the ticket on your phone, or a printed copy, or in the app. The guard may use a scanner to scan the barcode on the ticket.
When e-tickets are available, this option has replaced the Mobile Tickets and Print Your Own delivery options. We like to think it combines the best of both of the previous options, giving you more flexibility then before.