If you’re receiving a call and your friend is a contact, you’ll see a video preview of the other line before they can see you. And if you’re calling someone, Duo will let you know that everything you’re doing can be seen by the recipient, so don’t try to pick your nose before they pick up!
If you're on Android, you can see the caller even on the lock screen. iOS doesn't give devleopers access to the lock screen in this fashion, but if you have your phone unlocked and the app ready, you'll see the caller before you decide to answer.
This feature only works with people in your contacts, so random strangers can't flash you. And of course, you can disable it in the settings if you prefer.