Cinematic film movies are shot at 24 progressive frames per second speed. A Frame is the smallest unit of a 24 fps FILM format. NTSC video is a "field-based" format of 59.94 fields per second. A Field is the smallest unit in interlaced video format. 2 fields make up 1 frame. So, this 59.94 fields per second equals 29.97 frames per second. 1 second in FILM (24 frames) is NOT equal to 1 second in NTSC Video (29.97 frames).

To be able to match the speed of an NTSC Video, conversion from a FILM format to an NTSC Video format undergoes a process called 2:3 pulldown or TELECINE. This process, in simplest terms, means "to add 6 frames so that a 24 fps becomes 30fps which is close to 29.97 fps (another trick is used to get to 29.97). Picture of the process - doom9.org