These are the
symbols used by the ancient Romans to represent numbers, and can
be seen, for example an old clock faces. Often they can be seen on
old buildings to indicate when they were built, or an old tombstones
giving a person's date of birth. The Romans did not use a
place value system.

The basic symbols and the numbers they represent are

I = 1, V = 5, X = 10, L = 50, C = 100, D = 500, M = 1000

Other numbers are obtained by placing these symbols beside each
other, and the order in which they are placed indicates whether
they are to be added or subtracted, as can be seen on a clock face.
Below mentioned examples will help out in translating the Roman
Numerals. Let see few of them:

If smaller numbers
follow larger numbers, the numbers are added. If a smaller number
precedes a larger number, the smaller number is subtracted from
the larger. For example, if you want to say 1,100 in Roman Numerals,
you would say M for 1000 and then put a C after it for 100, in other
words 1100 = MC in Roman Numerals.

Some more examples:

VIII = 5 + 3 = 8

IX = 10 - 1 = 9

XC = 100 - 10 = 90

MCMLXXXIV = 1000 + (1000 - 100) + 50 + 30 + (5 - 1) = 1984

And,

IV = 5 - 1 = 4 whereas VI = 5 + 1 = 6

IX = 10 - 1 = 9 while XV = 10 + 5 = 15

MCXXVII = 1000 + 100 + 10 + 10 + 5 + 1 + 1 = 1127