key frame interval
A key frame is a frame that stores a complete image. Key frames provide the entire image in the video stream; after that image is provided, incremental changes between related frames are stored (not the entire image). Thus, when the image in the video changes dramatically—for example, when switching between images—a keyframe must be created to mark the location of change. It’s recommended that you allow Compressor to determine the key frame interval for your output file; however, you can also set the key frame interval manually.
Note: More key frames make the output video easier to move through (scrub) during playback. Fewer key frames make the transcoding process faster.