Nick describes Daisy as stifled, because society and circumstances force her to be so disingenuous that she desperately wants to express true emotion, or “come out.”
This conversation takes place with Daisy, Tom, Nick and Jordan at the table. Dinner is a very false construct. You have to sit there and pretend to be proper and civil, no matter the circumstances. When Daisy compares Nick to a rose, he says he is nothing like a rose.
“I love to see you at my table, Nick. You remind me of a—of a rose, an absolute rose. Doesn’t he?” She turned to Miss Baker for confirmation: “An absolute rose?” (ch 1)
Daisy says this when Tom leaves, as if it “quickened something within her.” She is reacting to the fact that Tom was called away from the table. Jordan tells Nick that it is because Tom has a mistress. Nick realizes he is way in over his head with these people.
By virtue of being family, Nick gets thrust into the world of East Egg. He is not ready for it. Society he is not. He is stunned when Jordan tells him Tom has "some woman in New York" as if it is nothing and even more surprised when Daisy and Tom return to the table and act as if nothing is happening. Nick is aware that Daisy is stifled about ready to burst, and the reader is ushered into this strange world with him.