iPhoto displays this query when you’re looking at a photo containing a face that iPhoto thinks it recognizes, but that you haven’t yet confirmed.
To confirm iPhoto’s suspicions, click the check mark. If iPhoto misidentified the person, click the X.
This process is simply a convenient alternative to switching into the Confirm Additional Faces mode and confirming or rejecting the photo there. It’s iPhoto’s way of saying, “Hey, as long as you’re here, maybe you want to confirm this person’s identity.”