Charms are innately-magical single-use magic items
Charms are one-use magic items which grant specific bonuses or mimic other channeler spells. They are formed from ancient rituals and natural forces, and are therefore considered “innate” rather than “channeled” magic, and are much harder for astiraxes to detect.
Charms can look like almost anything, and are therefore easy to transport and conceal. Legends might exist of truly powerful charms, but the most common ones by far mimic the function of simple spells (spells with few or no prerequisites). Once used, the charm becomes inert.