The trick with optimising any content is to figure out the main point of the video and think of all the keywords around that point. Once you have a few keywords, use the Google keyword planner to find out how many average monthly searches each one gets. Then pick the one with the most searched and use that keyword to optimise the video.
Ideally you should have a set of popular keywords in your mind before you even create the video but sadly this is not alway the case and optimisation is often an after-thought. Thought of at the earliest stage, the script in the video can be tweaked slightly before shooting to mention the relevant keywords and will untimatly increase the chances of gaining valuable organic traffic.
Optimising basically means to make it clear to search engines, what your content is about, helping that piece of content appear higher up in search engine results. YouTube is a search engine, therefore we have to make it clear to YouTube what are videos are about.
You need to make sure that your chosen keyword is included in the title of the video, at least twice in the first 4 lines of the video description and in the tags of the video. This will help that video appear higher up in the YouTube search results.