The description box looks like an afterthought and behaves like real estate. Its top two lines are ad copy, shown before anyone clicks "more". Its next couple hundred words are how search understands the video. Its bottom is for links nobody reads until they need them. Three jobs, in that order, and writing them as one undifferentiated blob wastes all three.
Most descriptions are either empty or a wall of links and hashtags. Both leave value on the table: the empty one tells search nothing, and the wall buries the two lines that actually get seen. A little structure fixes both, and it takes five minutes once you know the shape.
The three layers, top to bottom
The top two lines are the only part shown above the fold, before the "more" cutoff. Treat them like ad copy: a sentence that reinforces the title and gives one more reason to keep watching. Do not waste them on "In this video..." or a row of links.
The next two hundred words are for search and context. Describe what the video covers in natural language, using the words people would search for, written for a human rather than stuffed with keywords. This is the part that helps YouTube understand and surface the video.
The bottom is for everything else: timestamps if they are not elsewhere, related links, your socials, the housekeeping. Useful, but nobody reads it until they want something specific, so it goes last.
Write for a person, not a keyword stuffer
The old trick of cramming the description with repeated keywords does not work and reads as spam to the humans who open it. YouTube understands natural language well, so a clear, honest description that happens to use the relevant terms does the search job without the cost. If a sentence is there only to repeat a keyword, cut it.
One genuine, well-written description also becomes raw material later: the first lines can seed a Community post, the summary can feed a newsletter blurb. Written once, properly, it earns its keep in more than one place.
Where Chewbr fits
Write the description is step 25 of the 47, between the title and the chapters. The chapters that come next actually live inside this box, which is why the two steps are neighbours.
Keep reading
With the description written, add the chapters inside it to open a search surface. The top two lines echo your title, and the searchable middle uses the words from keyword research.