'Private Galleries' enable you to give access to certain photos you don’t wish every member on the site to be able to see. You give access to your private gallery by checking the appropriate box which will pop up when you click the padlock symbol when sending a message to another member (this option only becomes available once there are photos in your private gallery).
To add a photo into your private gallery go to 'Photos' -> 'Add Photo' and choose the private gallery option.
Anyone can see your private gallery if you give them access, this includes Standard members.
Learn more about Private Galleries, here.