35 mm film is a type of film gauge used for moving pictures and photography, which could be developed and projected. This was invented by Thomas Edison in the late 1800s.
It remained the standard and dominant format in shooting film until the digital era. It had better quality, was internationally known (cinema projectors could only project in that format) it could incorporate sound and colour and it was more cost efficient than other competing formats.
35 mm film was important for the film industry, as it was the first widely used high-quality format that could be processed internationally.
Even today with digital cameras, the same principles and style of 35 mm is used.