Ember guards stop burning embers entering through gaps greater than 3mm. They're made from non-combustible mesh with a hole size of 2mm or smaller, and they're mandatory for new homes in bushfire-prone areas with a BAL rating of 12.5 or higher (AS 3959-2009).
For BAL 12.5, 19 and 29, ember guards can be corrosion-resistant steel, bronze or aluminium. BAL 40 and FZ are limited to corrosion-resistant steel or bronze. Most generic plastic 'gutter guards' don't qualify as ember guards - use a correctly specified product.