Each image which has at least one face detected will be given a focus score. This score is based on a number of factors including individual subject focus scores, their hierarchy and the image's context.
You'll see the focus score for each image in the bottom right above the Image Assessment indicator. If the focus score icon is blue this indicates that this image has the highest, or joint highest, focus score in that particular scene.
You can also filter your gallery by focus score. For example, you can set the focus score filter to only show you images with a focus score of 6 or above, plus images without any score.