Here's a breakdown:

* NH₄⁺: Ammonium ion

* Al³⁺: Aluminum ion

* SO₄²⁻: Sulfate ion

* ·12H₂O: 12 molecules of water of crystallization (indicating the compound is a hydrate)

This formula represents a specific compound known as alum, which is commonly used in water treatment, papermaking, and other applications.