When you start the app it scans the nearby area for doors and displays them. There you can see the name of the door (for instance "Office" or maybe "Garage") and there is a button labeled "Unlock" that lets you unlock the door.
When you press the button the app begins to unlock and when you see the green icon the door is unlocked and you are free to open the door.
Should the app fail to unlock a door a yellow icon is displayed. When that happens you can try to move closer and press the "Unlock" button again when it's available
Move closer to the door then press the orange button in the bottom right to scan the surrounding area again.