Definition of Metaphor
Throughout the latter part of the novel, Lily uses a metaphor to describe her ideal man for Rosemary: she says that Rosemary needs an "anchor," someone who is steady and can hold her down, to counter her flighty nature. Lily believes that Rosemary needs someone reliable to have a "normal" marriage, have children, and settle down, as this is the ideal to Lily. Despite Lily pushing against the standards for women at the time, she still believes that the ideal marriage involves a steady life in one place, and she worries about Rosemary's future without someone who can balance her out:
But I did have a good notion of the type of man she needed—an anchor. That girl still had a tendency to be flighty, but with a solid man beside her, I could see her settling down, teaching elementary school, raising a couple of kids, and dabbling in painting on the side.
She emphasizes this even when Rex enters the picture. Lily sees Rex as even more irresponsible than Rosemary and understands that their marriage will be tumultuous, but Rosemary will not listen to her. Rex, too, refuses this metaphor, believing that Rosemary deserves more than to have a relationship mirroring Lily and Jim's:
I shook my head and looked at the lilies. “I could cut you all the slack in the world, but I still think my daughter needs an anchor.”
“The problem with being attached to an anchor,” [Rex] said, “is it’s damned hard to fly.”
Lily knows that Rex cannot give Rosemary what she thinks Rosemary needs, and Rex pushes back against this, saying that what Rosemary wants is not what Lily wants for her. Lily wants to "fly," something that is impossible if she has an anchor. That is the point of what Lily wants for her, however: she knows that Lily will not settle down on her own, and she wants someone who will prevent her from flying away.
Eventually, Lily accepts that Rosemary was the one kid that she couldn't teach, and this is how her life will be, even with all the instruction in the world. Rosemary is too headstrong to listen to her mother, much as Lily was when she was younger.