She's started to pretend to fall asleep after they have sex.
Robin wonders how long Barney's been talking to her in her sleep. How long he's been talking to her like this. Like she's The One. Robin reads Barney better than he thinks she does-she knows when he's being honest and vulnerable-and even though the first time she woke to hear him telling her all of these dreams, she'd never heard that tone of voice coming from him before, she knew what tone of voice he was using.
Some nights, Robin's not sure why she doesn't say anything about it, show Barney that she knows all about his dreams. But Robin knows that if she outed him, Barney would be hurt. He would stop sharing these little pieces of himself and Robin thinks he needs these fantasies as much as he needed little stories to help him while he was growing up.
She doesn't want Barney to stop telling the stories, and not because she maybe sort of likes to hear them told. Robin just wants Barney to be okay and feel like there's a future where a girl doesn't leave him feeling hurt and inadequate.
And when she falls asleep to the sound of his voice, Robin dreams of holidays in a warm house with kids running around and teasing Barney warmly about cooking, of warm beds and waking up to the face of the man she's with now for the rest of her life with smiles.
Of course, they're just dreams. One day, they won't be Barney-and-Robin. But for now, it's nice to dream.