It's well known that the Romans used Latin letters for writing numbers.
It is considered that the Roman numeral system is a classic example nonpositional numeral systems, i.e. the numeral systems in which the value of a figure is independent of its position.
We remind you that in Roman numeral system I is 1, V is 5, X is 10, L is 50, C is 100, D is 500, M is 1000.
For example, number 3, is written as III in Roman numerals.
Although, everything is not so simple and it's a nonpositional numeral system because there is an additional rule that modify the value of a digit according to its place.
That rule forbids the use of the same digit 3 times in a row. That's why 3 is III but 4 is IV and I(1) placed before the larger digit V(5) means subtraction so it's actually -1.
Anticipating the obvious question, we must say that the largest number allowed in this calculator is 3999. For larger numbers, which were used mainly in Medieval time, several different notations were used, including apostrophus and vinculum, but none of which were ever standardised.
Here is two calculators below - for numbers ranging from 1 to 3999 to roman conversion and vice versa.