Playback is a more specific term that simply means to play the media, in your case a Youtube video. It is used to mean the actual action of the video being played from beginning to end rather than the more general 'starting' or 'showing' the video. The speed of the entire video play time is controlled rather than just the beginning. In this case it is being more specific because it is describing the speed at which the video is displayed from beginning to end. So in general it means the act of playing the video. I hope this helps. Mat

Hi Mat. Thanks so much for the detailed explanation. I still have one follow-up though. Could I say "let you change your playing speed"?

To play a video or replay a video can mean the exact same thing or different things. To play a video can mean for the first or hundredth time. Yet to replay the video definitely wouldn't be the first time. Playback can mean the first time in some contexts. It is more general than replay. Replay would be hard to use for a first time viewing, but playback speaks more to the action of playing than the number of times a clips has been seen. The actual playing itself is altered by the speed control here. That is why the simpler 'play' was not used. Play is general, playback is more specific to the action of playing, and replay would be even more specific and pointed toward a subsequent viewing. Mat