World Cup rules/etc question-help please

[harmonica]

....so I see some possible foul/etc in a game....but the refs do not call it....the announcers say the refs on the sidelines are reviewing that possible play/foul.......but the game continues on......what is happening?? are the refs really reviewing the play/foul?? but the game is going on.....?? what happens if they overrule the field refs?? or am I wrong about this??
thanks all replies