In a word, nothing. Film production and principal photography are essentially interchangeable terms so long as the person using them understands the distinction between film production and the other stages of filmmaking, such as pre-production and post-production.
Film production is sometimes used as a catch-all phrase for the entire process of making a movie. However, using the term in this way can cause confusion for others who understand it correctly as principal photography.
As long as you use the term film production to denote the creative execution phase of making a movie, you can use it or principal photography when talking about it with others.