Chances are that when you search for a video on Google or at YouTube, the results that you receive are based upon text about the video rather than the content of the video itself. The search algorithm involved might look at the title of the video, as well as a description and tags entered by the person who uploaded the video as well. Annotations on the video may also play a role in determining what terms and phrases the video may be determined to be relevant for as well.
For example, the video below announces Google’s new food recipe search option, and provides a detailed description about the new feature. But none of the text accompanying the video mentions that the person providing details about Google’s added functionality is one of Google’s executive chefs, Scott Giambastianai. If you search for [Google executive chef], you wouldn’t see this video appear in YouTube’s search results and you probably should.