Barcode Scanner continuously scans a square region shown on your screen — just line up the phone so the barcode is completely inside the viewfinder rectangle:
When a barcode is read, a beep sound will play and you'll see the results of the scan, a description of what the barcode contains, and options to take action on the contents.
If you're having trouble scanning, make sure to hold the phone steady. If the camera is unable to focus, try moving the phone further or closer from the barcode.