Look at the example image taken from YouTube
When I watch video file or listen to audio file, there is always a horizontal like which shows how much video/audio has been cached as well as my current location in video/audio.
What is this line called in plain English?
The most common term for it that I know of is seek bar, though some media players (like VLC) call it track slider.
In audio/video editing terminology, the word used for it is timeline. A few related terms include the playhead, which is the symbol that marks your current position on the timeline, and scrubbing, which refers to moving the playhead manually across the timeline.
You are only going to get opinions with your question. There is no 'set' word, it's just how different people know it / refer to it.
Personally, a progress bar or timeline. I assume timeline would be more accurate of the two.
You may want to ask https://english.stackexchange.com/ as this is for English.
I have heard it called a "Buffering Bar" on many occasions.
Dave is right though in saying that it will vary from person to person.
