The best way to write the description text for your YouTube video SEO is to write in that box exactly what it’s titled: the description of the video. Be sure to include keywords that are relevant to your video without stuffing too many. Mentioning some of your keywords once, twice, or three times max is totally sufficient. In fact, if you do it more than that YouTube and Google may consider your keyword optimization to be “keyword stuffing.”

Watch the full discussion here.