It can refer to:
* A biography or autobiography of a person who has dedicated their life to the world of cinema. This could be a director, actor, screenwriter, producer, or any other person who has worked in the film industry.
* A film or documentary about the history and evolution of cinema. It could explore the different eras of filmmaking, the influence of technology, the changing role of audiences, or the impact of cinema on society.
* A more abstract concept of living a life that is deeply intertwined with cinema. This could mean someone who is passionate about films, watches them constantly, or sees their own life reflected in the stories on screen.
Without more context, it's difficult to say exactly what "Una vida en el cine" means in a specific instance.