First championed by Franz Boas to challenge the attitude that Western culture was inherently superior to others, which important tenet of anthropology states that a society's customs and ideas should be described objectively and understood in the context of that culture?

Cultural relativism.

Explanation:

The concept of cultural relativism was first established by anthropologist Franz Boas. Its basic concept is that people's distinct customs and values should be studied and understood according to their own cultural system.It suggests that a culture should not be judge according to the values of another culture. It advocates that each culture is unique and comprise its own distinct values and customs.Cultural relativism rejects the concept of ethnocentrism and thus, challenge the Western's attitude of believing its culture as inherently superior to others.