Korean chili flakes
Dried, coarsely ground Korean chilies without seeds, bright red, mild to medium hot, fruity and slightly smoky.
Gochugaru are dried red Korean chilies that are seeded and ground into flakes or a coarse powder; a coarse grind is used for kimchi and a fine one for sauces and pastes. The heat is noticeably milder than that of ordinary chili flakes, the flavor fruity, slightly sweet and often a little smoky, and the color a vivid red. Precisely because it is mild, it is used in large amounts that give a dish color and body without making it unbearably hot. It is sold in Asian grocery stores and online, usually in larger bags; once opened, it keeps best tightly sealed in the fridge or freezer.
Substitute:
The closest match is Aleppo pepper or pul biber from a Turkish grocery, which is similarly mild and fruity but can be oilier and a little saltier. A mix of sweet paprika with a small amount of cayenne matches the color and heat, but not the fruitiness or the coarse texture. Ordinary chili flakes are not a good substitute, because in the quantities a Korean recipe calls for they become far too hot; for kimchi, real gochugaru cannot really be replaced.