has multiple meanings depending on context:

1. Medicine / Drugs (약 as a noun)
약을 먹다To take medicine
감기약Cold medicine
두통약Headache 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 지키다To keep a promise
약속을 하다To make a promise