Naomi's other, not-as-awesome daughter-in-law, Orpah, was married to her son Chilion before his untimely demise. Though she wants to leave Moab with Ruth and follow her mother-in-law to Judah, Orpah changes her mind when the going gets tough.
At first, Orpah cries along with Ruth and pledges her loyalty to Naomi. But, then, when Naomi starts to lay out a way more depressing future for them in Bethlehem—"the hand of the Lord has turned against me" (1:13)—Orpah is convinced. Peace out, ladies.
Orpah is mainly hanging around to be a handy dandy little contrast to the behavior of her fellow Moabite, Ruth. Sure, Orpah would love to stay with Naomi (she's mother-in-law of the year), but the road ahead just isn't looking so sunny. A regular person—Orpah—would turn back when it comes to hardship like that. But Ruth isn't a regular person. She's super loyal and all-around amazing. Which is why it's called the Book of Ruth and not the Book of Orpah.
Like Mahlon and Chilion, Orpah's kinda been set up to fail right from birth. Her name means "back of the neck" (source, p. 241). As in, what you see when someone turns to walk away from you. Harsh, Orpah's mom. Real harsh.