Some games (such as the Final Fantasy and Dragon Quest series) use Roman numerals in either the titles of the games or in the game itself. Here is a quick, simple resource guide to help you get a basic understanding.
I‘s indicate single numbers, so they should be added up. Example:
II is two
III is three
However, if an I appears before a letter, (V=5) you minus one. Example:
IV is four
If an I appears after a letter, you add one. Example:
VI is six
It should also be noted that X=10.