Gochugaru is Korean chili pepper flakes or powder. It is a quintessential pantry item for Korean cooking. The spice level of these hot pepper flakes varies from mild to hot. Maangchi who is the author of Maangchi’s Real Korean Cooking: Authentic Dishes for the Home Cook recommends the ones that are made with sun-dried peppers, which will be labeled as 태양초 on the package.
For longer shelf life, keep the gochugaru in the freezer after opening to maintain its freshness.