Why did she put "supposed to"? Can't she just say "What is that mean?"?
Answer from a Native speaker
"Supposed to" in this context is used when one is annoyed or angry with another by what he or she said. Allie is upset that Noah said the only reason she is with Lon is because he provides her with the security of money. If Allie just said "What does that mean?" the line would lack the emotional value of how offended and angry she is with Noah. Here is another example of "What that's supposed to mean?" Person 1: You aren't ready to compete against her. Person 2: What's that supposed to mean?