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:
1D barcodes like those found on products require a phone with autofocus. Without it, only QR Codes and Data Matrix codes will be scannable.
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.