In F. Scott Fitzgerald's The Great Gatsby, in Chapter Seven, Daisy (who is married to Tom Buchanan) has been regularly visiting Gatsby in the afternoons, carrying on an affair. Gatsby has fired all his servants to keep them from gossiping.
In this particular scene, Nick (the narrator) and Gatsby have come over to the Buchanans' house. When Daisy begs for cold drinks—because it's so very hot—Tom goes to get them, and then Daisy kisses Gatsby...
As [Tom] left the room...[Daisy] got up and went over to Gatsby and pulled his face down, kissing him on the mouth.
"You know I love you," she murmured.
Tom soon returns with drinks. He shows the men around, and eventually everyone sits down to lunch. When they have eaten, they try to decide what to do with themselves for the afternoon. Daisy suggests that they go to town.
"Who wants to go town," demanded Daisy insistently. Gatsby's eyes floated toward her. "Ah," she cried, "you look so cool."
Their eyes met, and they stared together at each other...
"You always look so cool," she repeated.
She had told him that she loved him, and Tom Buchanan saw.
..."You resemble the advertisement of the man," she went on innocently. "You know the advertisement of the man—"
In this part of the story, it is literally very hot. The lights have been turned down in the house and no one feels like moving around. However, Daisy has just told Gatsby that she loves him. She may also be inferring to her passionate nature by noting how "cool" Gatsby is: he is just what she needs to tame the fire burning inside her. Her reference to the "advertisement of the man" may refer to the perfect man, for this is what advertisements often try to convey to their target audience—that the man or woman in the ad is perfect in some way.
In essence, while the temperature may be hot, Daisy is also "hot"—for Gatsby—and the only thing that will satisfy her is time spent with him. Comparing him to the advertisement is letting him know that for her, he is absolutely perfect.
This behavior makes a clear statement to Daisy's husband. Though they don't speak of love aloud in his presence—Gatsby and Daisy are having an affair...much the same way Tom is. Nick Carraway notes that Tom "was astounded." Looking at Daisy, he sees something very different in her.