What does it mean that there is a small mouth moving on the screen when taking pictures with a mobile phone?

If the small box that moves by itself is facial recognition, it can automatically identify the position of the face and then focus. Make sure the position of the face is the clearest. Have you noticed that it always runs in people's faces?

If you don't move it yourself, the box pointed by your finger is the focus. Click there to get the most attention.