In different countries, photographers have different ways of trying to get the people in the picture to all do the same thing at the same time. In the UK, for instance, people say "Say cheese". This is because, in order to say the word "cheese", it is necessary to move the mouth in such a way that it looks like a smile. Therefore, if all the people in the photo say "Cheese" at the same time, the photographer is likely to get a photo in which it looks like they are all smiling.
I have no idea what the photographer in your example might have been saying, that sounded like "Say levels".