그럼 그냥 형이라고 해
English translation: "Then just call me Sang-woo."
Literal translation: "Then just call me older brother."
In Korea, there are honorifics people use to refer to others based on age, gender, and social status. Most people don't call each other by their names unless they are close friends at a similar age.
For example, there are specific words to call an older brother, older sister, younger brother, and younger sister. These words also depend on which gender you - the speaker - identify as.
This moment is particularly important because Sang-woo tells Ali to call him "hyung" - the word for older brother from a younger brother. In Korean slang, you don't necessarily have to be related, but it's always an honor to have someone tell you to call them "hyung."
For most of the show, Ali calls Sang-woo "sir." The transition into "hyung" represents a big step in the relationship between Sang-woo and Ali, especially since Ali has been an outsider for so long.
This also makes it a lot worse when Ali is calling Sang-woo "hyung" as he realizes Sang-woo betrayed him.