Here's how to understand the formula:

* Fe represents the element iron.

* (III) indicates the iron has a +3 charge (ferric ion).

* SO₄ represents the sulfate ion, which has a -2 charge.

* ₂ indicates two iron ions are needed to balance the charges.

* ₃ indicates three sulfate ions are needed to balance the charges.

This gives us a neutral compound with the correct ratio of iron and sulfate ions.