What is this flat metal sieve that came with my pressure cooker for?
The sieve-like item is a steamer insert, used to cook things that do not cook well if submerged in the comparatively large amount of water a pressure cooker needs to operate. The wire is a stand for this insert, to raise it above the waterline.

Unless you bought it used: Pressure cooker makers tend to give free books away with their pots, often a classic about a spanish guy called Manual. Good reading while watching a pressure cooker.