Listen to Madonna's song 'Material Girl' for an explanation.

A female who likes expensive things, usually designer. They have no interest in the lower-end of the market and unbranded items.

ha ha, This is a funny, but good question. "She's a material girl living in a material world." One thing that hasn't been mentioned is material is used in contrast to spiritual. Someone who meditates, prays or attends a church or temple would be more focused on their spiritual life. One who never meditates and instead focuses on food, money, partying, and physical pleasures is focused on their material life.

I think it is specifically a reference to the Madonna song, "material girl." Generally, "material" can be used as an adjective, often in a religious context, to mean the opposite of "spiritual." Material things mean things like money and luxuries. A clergyman might complain that we "are living in a material world." In the lyrics of the song, the singer says "the boy with the cold hard cash/Is always Mister Right, 'cause we are living in a material world, and I am a material girl." She is only interested in money, and justifies it by saying that is the way things are today.