Link text is the visible text inside a link. This text tells users and Google something about the page you're linking to. Links on your page may be internal—pointing to other pages on your site—or external—leading to content on other sites. In either of these cases, the better your anchor text is, the easier it is for users to navigate and for Google to understand what the page you're linking to is about.
Completion rate: Completion rate is the number of people who completed your video divided by the number of people who played it. Completion rate, and other engagement metrics, are a great way to gauge a viewer's reaction to your video. Do you have a low completion rate? Are people all dropping off at a certain point? This might be a sign that your video content is not resonating with your target audience.