The "sound barrier"
is a zone of intense turbulence
that that is created as a plane (or whatever) accelerates to the speed of sound.
This is not hard to visualize if you recall that sound is caused by pressure waves propagating through the atmosphere. As long as a plane is subsonic, the pressure waves caused by its flight can escape ahead of it. Problems crop up as it becomes transonic - that is, just as it nudges against the speed of sound. The pressure waves produced at the leading edges of the craft can no longer outrun it, and they begin to "pile up", creating a wall of pressure that the plane must pass through to become supersonic. The forces involved are strong enough to tear apart a craft designed for subsonic flight. Once the plane has exceeded the speed of sound, and has entirely passed through the pressure wave, things smooth out again.