When adding images to an image field, you'll sometimes get the option to add a focus point to an image. When you get this option, you'll see a gear icon underneath the image (see 1 in the screenshot below).
Clicking on that gear icon, a pop-up will open which will show your image in multiple formats. By clicking in the top left picture on the spot you want the image to focus on, the white circle will move to that position (see 2 in screenshot below). You will immediately see the change this triggers in the different format previews.
The white circle marks the spot the image will be focused on when resizing the image. This can happen when a user resizes their browser or the image is used in multiple formats throughout the website. This function is mostly used on hero images which you will learn about more later.
For example: when adding a hero image that depicts a person, you can focus the image on the persons face by moving the white circle there. That way, whenever the image gets used in a different format, the persons face will likely never be cut off.