Slider video

I was watching a YouTube video about the guardian theme that showed videos in the slider, I have uploaded videos and loaded them but they will not show on the website (blank slider other than the text).

Are there certain formats or lengths of videos that need to be used? (I’m using a 13s video and am about to try a shorter one, I have tried a few formats)

Or have I been mislead? It was part of my reason for deciding on guardian in the end

Thanks!