Currency used at the time of Muhammad. For general historical background, see for example the Wikipedia entry on Dirham.
Dirhams are mentioned in the Qur'an in Surah 12:20 as being the payment when Joseph is sold into slavery. This is, however, a severe anachronism and historical error in the Qur'an since Dirhams did not yet exist at the time of Joseph. In fact, the very first coins were only invented several centuries after his time.
For a detailed discussion on this topic, see the article