약 has multiple meanings depending on context:
1. Medicine / Drugs (약 as a noun)
약을 먹다 → To take medicine
감기약 → Cold medicine
두통약 → Headache medicine
2. Approximately (약 as an adverb)
약 10분 정도 걸려요. → It takes about 10 minutes.
약 100명 왔어요. → About 100 people came.
3. Promise (약속, shortened form)
약을 지키다 → To keep a promise
약속을 하다 → To make a promise