A keyframe (an IDR frame in H.264/HEVC terms) is a frame encoded with no dependency on any other frame, so a decoder can begin decoding from it cold. Every GOP starts with one. Keyframes are the only points where playback can start, seek, or switch renditions.
Their placement is a balancing act. Frequent keyframes make seeking and ABR switching responsive and segments cleanly splittable, but they cost bitrate because they cannot be compressed against neighbours. For adaptive streaming, keyframes must be aligned across renditions at segment boundaries so the player can switch quality invisibly.
