Basically, the meaning of 天丼 (tendon) is the shortened form of 天ぷら丼 (tenpura donburi), which means "bowl of tempura". It is as simple as a bowl of rice with various tenpura on the top, but it is really delicious. This restaurant has a long history, so it is only natural that they have special ways of preparing their dishes.
This recipe was introduced by the Portuguese missionaries in Japan during the 16th century, in the city of Nagasaki (長崎).
Simple enough right? Sometimes a good dish doesn't need too much complex cooking to be delicious!