H.264 is a video compression codec standard. It is ubiquitous - internet video, Blu-ray, phones, security cameras, drones, everything. Everything uses H.264 now.
H.264 is a remarkable piece of technology. It is the result of 30+ years of work with one single goal: To reduce the bandwidth required for transmission of full-motion video.
Technically, it is very interesting. This post will give insight into some of the details at a high level - I hope to not bore you too much with the intricacies. Also note that many of the concepts explained here apply to video compression in general, and not just H.264.