Hello!
While watching a video, it is often helpful to take notes on any important concepts or definitions that are explained. In addition, any smaller details that pop out may be useful for any assignments/tests that you are preparing for.
To site a youtube video, in an essay for example, you also need to state at what time the concept/definition was explained. If a definition is stated in the video around 2 minutes and 25 seconds, you would need to cite that the definition is mentioned at 2:25.
I hope this helps answer your question! Have a great day!