This is how you use Roman Numerals.
I is one
II is two
III is three
IV is four
V is five
VI is six
VII is seven
VIII is eight
IX is nine
X is ten
L is fifty
C is one hundred
D is five hundred
M is one thousand
The basic rule is smaller symbols before larger symbols are to be subtracted, and smaller symbols after larger symbols are to be added.
As you can see, if you put the I before the V, it is the same as subtracting one from five, which equals four.
When you put the I after V, it is adding one to five, to make six.
XII is twelve.
When the British use these symbols for the monarchy, we would say VI is the sixth. In this case George the Sixth.
The reason for the slight wording change of the number is because George VI is the sixth George to be named within the royal family. So although the sentence will be written as VI, you would say it when speaking as "the sixth".
For Elizabeth, it would be Elizabeth the Second.
These are used from a hold over from the Roman Empire when it ruled europe and most of Britain. You can also look up about Roman Numerals on the internet, as they are still in use.