Here's how we arrive at that name:

* Cr is the symbol for chromium.

* O is the symbol for oxygen.

* The subscript 2 indicates there are two chromium ions in the compound.

* The subscript 3 indicates there are three oxygen ions in the compound.

Since oxygen has a charge of -2, and there are three of them, the total negative charge is -6. To balance this, the two chromium ions must each have a charge of +3. This is why we use the Roman numeral (III) in the name to indicate the charge of the chromium ion.