When a bar appears over a Roman numeral, it indicates that the number should be multiplied by 1,000. This is a concept used in Roman numeral notation to express larger numbers.
For instance, if you see a bar over the Roman numeral X (which represents 10), it would indicate 10,000 (10 multiplied by 1,000). Similarly, a bar over VII (which represents 7) would denote 7,000. This notation helps to easily represent large values, as the traditional Roman numeral system has limitations in expressing big numbers without resorting to more complex combinations.
So, in essence, the bar over a Roman numeral serves as a multiplier, significantly increasing the value of the numeral it overlays. This practice dates back to ancient times and has been a part of the Roman numerical system to accommodate the need for larger representations.