In my opinion, "playback" is just like "replay", am I right? But mostly people say "playback speed". I think they mean the play speed, why the "back" should be added behind "play"? What is the difference?
No, they are correct when they say playback.
In order to help you understand it, I'll try to explain how I intuitively understand it.
When you record something on a media, you are effectively putting stuff on it (video, audio, other files, ...). Then, at the moment when you want to consume the data, you will need to operate the device in order to make the device "give the content back to you" (your eyes, ears, or any other non-standard limbs you might have ;) ). So play-back, which implies that earlier in time a contrary operation (storing the content) took place.
Also usually we store stuff in an ordered way (i.e. a video is a set of many pictures ordered in time), so the expression "play it back" perhaps also comes from the implicit understanding that we need to go back to the start of it (yes, once you had to actually rewind a tape), before being able to play it..
"Playback" means :
the reproduction of previously recorded sounds or moving images.
This simply means "playing" an audio or video that was recorded earlier. Now the "back" can be edited in most contexts. It is simply used to show that the track was recorded earlier, or "long back". It's a form of replaying a piece of recording. Maybe that's why "back" is used.
But there are occasions where you cannot replace "playback" with "re-play" or "play":
He was a renowned playback singer, back in the 80's.
Now, you cannot say 'he was a "play" singer'. That just sounds odd. Technically, it means the same - You record. You play, but they cannot be interchanged in all contexts.