No video when playing mp4 movie in vlc

[vno]

Consider this:
http://askubuntu.com/questions/668834/vl...udio-works

I had the same issue: sound but " No video when playing mp4 movie in vlc".
And this setting (from the link):
Remove the tick for Accelerated video output (Overlay)
+
Change Output to X11 Video output

Solved it.

(now I'm wondering what is this X11 video output...?).