A GOP — Group of Pictures — is a chunk of video that starts with a full-quality keyframe (an i-frame) and ends just before the next one. Inside, every frame is described as a small difference from its neighbours rather than coded from scratch. A typical streaming GOP is 1–4 seconds long; a typical movie at 24 fps with a 2-second GOP has 48 frames per group.
Why GOPs exist: they enable random access. When a viewer scrubs to the 14:23 mark of a film, the player can't start decoding from any random frame — most frames depend on earlier ones. Instead, the player jumps to the nearest GOP boundary (the most recent I-frame) and starts decoding from there. The shorter the GOP, the faster the seek and the cleaner the channel switch in live TV. The longer the GOP, the better the compression — I-frames are big, and putting one in every second of video wastes a lot of bits.
In practice, GOP length is the single most important encoder setting after bitrate, and it's tightly coupled to delivery strategy. HLS and DASH segments must start on a GOP boundary, so the GOP length directly determines the smallest segment your platform can deliver (typically 2–6 seconds). Live broadcast and IPTV want short GOPs (0.5–1 second) so a viewer tuning in mid-stream doesn't see a long black gap. VOD on Netflix or YouTube can use longer GOPs (4–6 seconds) for bandwidth efficiency since viewers usually start at the beginning anyway. Pick the GOP based on how viewers will actually use the content.
