Daniel thinks he'd love to light Charlie on fire, but he tries to focus on Natasha's hand. Dae Hyun asks what he's doing in Korean, and Daniel makes sure to answer in English as he hands his dad the pouch. Charlie offers to translate the Korean for Natasha, and Dae Hyun reminds Charlie that he doesn't understand Korean. Charlie insists he gets by, and his dad asks if that's why Charlie failed out of Harvard. Charlie tells Natasha that his dad is calling him stupid. Daniel can tell his dad is unspeakably angry.
From Dae Hyun's perspective, the presence of one mean, failing son and the other with a non-Korean girl likely represents failure on his part to point his sons towards achieving the American dream while also preserving their heritage. Both Daniel and Charlie are going against their parents' wishes here, which shows that the Bae parents have a very narrow view of what success looks like.
Dae Hyun puts on his storeowner face and asks Natasha if she wants something. She tries to politely refuse. Dae Hyun leads them to the chemical aisle and offers Natasha a tub of relaxer, telling her it'll make her hair not so big. Natasha explains that she likes it big. Charlie makes an innuendo about the size of Daniel's penis, and Daniel voices the joke for the group. He throws the pouch at his dad and leads Natasha out, flipping off Charlie as he does.
Though the reader is intended to interpret Daniel's actions as positive changes and a signal that he's growing up and coming into himself, in the eyes of Daniel's family, this is an act of rejection (especially since Daniel isn't at his Yale interview where he's supposed to be).