Question

いろはす

赤線を引いたところのbetter offのoffはどんな概念を有しているか、また品詞は何か？という問いがあるんですが、全く分からないので詳しく説明してくださいお願いします…

His parents were poor illegal immigrants from Mexico. Henry, because he was
born in the United States, had U.S. citizenship. However, Henry's father had
trouble with the law and was frequently in jail. His mother decided that they
would be better off away from her husband and moved to New Mexico with her
six children. Henry was four years old at the time. In their new surroundings,
the family often had to share a living space with other poor families. Four
children often slept in a single bed. Henry's mother worked hard at two jobs