Aqua regia “dissolves” gold by oxidizing it to gold (III) ions (aka auric ions, Au³⁺), due to the oxidizing power of the nitric acid and the chlorine it produces by reaction with the hydrochloric acid.
Sodium metabisulfite in acid breaks down to form sulfur dioxide, which is a reducing agent. The SO₂ reduces the Au³⁺ to Au, and also reduces the nitric acid to nitrous acid and the chlorine to chloride ion, being itself oxidized to sulfate.
