The first two lines of the description are viewable on the results page and should therefore describe what the video is about.
The next line of the description is viewable under the video while it is playing, so here you should remind viewers to subscribe, link to your website or other videos you've created.
It is also a good idea to add time tags in your description. For YouTube to recognize these and add them as chapters in the timeline, the first time tag should start with 0:00. For example, 0:00 - Intro.
You can add Keywords as Tags; however, I do not believe adding tags affects your ranking, so there's no need to go crazy.
Linking to other videos during and at the end of your video, by way of an 'end screen,' is a great way of enticing viewers to watch more of your content.
