This only occurs with the ions formed by the transition metals. For example, Copper can form ions with a #1^+# or a #2^+"# charge. You would indicate the charge on the ion with a Roman numeral.
Consider #"CuCl"#. Since the chloride ion always has a #1^(-)"# charge, and there are no subscripts, we can know that the copper ion in this compound has a charge of #1^+"# in order for the compound to be neutral. We would name this compound copper(I) chloride, or in words only, copper one chloride. #"CuCl"# can also be named cuprous chloride, though it is not used very often today.
Now consider #"CuCl"_2"#. There are two chloride ions and one copper ion. The total chloride charge is #2xx1^(-)=2^(-)"#. The copper ion does not have a subscript and we know that ionic compounds are neutral. So in order for #"CuCl"_2"# to be neutral, the charge on the single copper ion is #2^+"#. We would name this ionic compound copper(II) chloride, or in words only, copper two chloride. This compound can also be named cupric chloride.
