Aonori is sun-dried seaweed, crumbled into flakes or powder or something in between. The name incorrectly suggests that they are simply crumbled nori-skins, but it is from a very different genus of seaweed. Aonori is made from (certain types of) sea lettuce.
Aonori is mainly used as a spicy garnish. It smells like mown grass and has the saltiness of seaweed. Nice as a finishing touch on for example okonomiyaki, takoyaki, noodle soup or noodles. You can also take a scoop in your tempura batter. Airtight for a long time at room temperature.
Source: learn all about Robin's website about Asian ingredients.