The scene occurs as Daisy is in the midst of rediscovering her love for Gatsby, and during Nick's courting of Jordan Baker. They visit and and Daisy tells Tom, her husband, to make them drinks; when he leaves, she kisses Gatsby.
"You know I love you," she murmured.
"You forget there's a lady present," said Jordan.
Daisy looked around doubtfully.
"You kiss Nick too."
"What a low, vulgar girl!"
"I don't care!" cried Daisy...
(Fitzgerald, The Great Gatsby, mrbye.com)
Daisy's love for Gatsby is in poor taste according to the norms of East Egg residents, even though it is somewhat common knowledge -- as Jordan tells Nick -- that Tom has a mistress. When Jordan jokes that she, "a lady" is present and so Daisy shouldn't show this affection, Daisy tells Jordan to kiss Nick, evening out the display. Jordan's reaction is repulsed, showing her East Egg sensibilities although she has been dating Nick for a while. This scene demonstrates how the growing tensions between the characters is damaging their relationships, and is the start of the first climax, where Daisy professes her love for Gatsby to Tom.
Daisy is cheating on her husband with Gatsby. When her husband leaves the room, she kisses Gatsby. When she's warned about her behavior by Jordan, Daisy tells her to kiss Nick.
The main idea that we need to understand from Daisy's attitude and also the attitude of the other characters centers on ethics and morality.
Daisy is behaving immorally. She is not only unfaithful to her husband but she has the audacity to show affection for Gatsby during the moment her husband is not there. And Daisy convinces herself that it's absolutely okay to behave this way because other people do it too. In fact, she is suggesting others behave in the same way (in this situation Jordan), perhaps again to reduce her guilt over her actions and justify them.