A GOP (group of pictures) is the structure of frames between one keyframe and the next: an independent I-frame followed by predicted P- and B-frames that only store the differences. Longer GOPs compress better (fewer expensive keyframes) but make seeking and bitrate switching coarser.
In adaptive streaming, GOP length must align with segment boundaries and be consistent across all renditions, because the player can only switch quality at a keyframe. A closed, aligned GOP structure is what makes seamless ABR switching possible; misaligned GOPs cause stalls or visible glitches at the switch point.
