What does it mean when there is a straight line over a roman numeral?

Answer 1

Over a whole number it was to ensure that it was understood as a number and not a word (also often underlined as well so the number appears between two lines);

Otherwise multiply it by 1000, for example:

_

V = 5 x 1,000 = 5,000

Answer 2

When there is a straight line (known as a vinculum) over a Roman numeral, it typically signifies multiplication. It indicates that the value of that Roman numeral should be multiplied by the number below the vinculum.