Why did VAR overturn Tim Ream’s yellow card during USA v Paraguay? - The strange moment occurred in the second half of the World Cup match in Los Angeles

In simple terms, a replay official is assigned before each match and monitors all "reviewable" decisions made by the referee on the field. If a potential "clear and obvious" error has been made, the on-field referee is alerted and may be asked to review the incident on a pitch-side monitor.