VAR, the Video Assistant Referee, is an official who reviews replays and advises the referee on the pitch. It can step in on four kinds of decision only: goals, penalties, straight red cards, and mistaken identity, and only to correct a clear and obvious error the referee has missed.
VAR was written into the Laws of the Game in 2018 after several years of trials, and the 2018 World Cup in Russia was the first to use it across a whole tournament. The referee keeps the final say, and usually makes it after watching the incident again on a pitchside monitor.
The threshold matters. VAR is meant to fix only clear and obvious mistakes, not to re-referee every decision, which is exactly where the arguments start, because what counts as clear and obvious is itself a matter of judgement.
The 2018 World Cup final showed both sides of it. France were awarded a penalty against Croatia after the referee was sent to the monitor to check a handball, Antoine Griezmann scored it, and France went on to win 4-2. The same review that settled that call is the sort supporters have argued over ever since.