When are roman numerals used when naming compounds?

The Roman numerals are used to show the oxidation number of transition metals because some elements have more than one possible oxidation state.

Some metals have the ability to form differently charged ions. For example, iron can form ##2+## or ##3+## ions. If you simply gave the name iron chloride, you would not know which charge the iron ion possessed.

A Roman numeral is to indicate the charge of the iron.

Iron (##"II"##) means the iron has a ##2+## charge Iron (##"III"##) means that the iron has a ##3+## charge

So, iron (##"II"##) oxide would have a chemical formula of ##"FeO"##. (The oxide ion has a ##2-## charge to balance the ##2+## of the iron to form a neutral compound.)

Iron (##"III"##) oxide would have a chemical formula of ##"Fe"_2"O"_3## (Here you need to find the common multiple of 6, so two iron ions with a ##3+## charge will balance the charge of three oxide ions with a ##2+## charge.)

Here is a video with added information about how to use Roman numerals in names of compounds.