Video chapters are the sections a video is divided into, each with its own timestamp, so viewers can jump straight to the part they care about from the player’s own progress bar. They are not a separate field of the video: they are generated from lines of text in the format 00:00 Chapter name written in the description.
In ōrbita
In a YouTube post, you write the chapters as lines of their own inside the description: you need at least 3, the first one has to be exactly 00:00, and each one must come at least 10 seconds after the previous one. Details in the YouTube guide.
Also known as: timestamps, YouTube chapters, time markers.