For me it's the thumbnail combined with the title. Usually, I know quite well what kind of content I am looking for, so I will do a Google or YouTube search for that and then look at titles first and thumbnail second. Usually, when I look for help videos I just outright assume that the top 3 videos will contain the content that I need to solve my problem.
When I look for gameplay videos I can go through a lot more of the results to look for a style of commentating that I like. And there the thumbnail also comes into play more because it determines how fun I expect a video to be.