He's talking about a time when he was little, and he overheard his brother saying that he wouldn't be able to do anything even if he tried, or something. I think it was to their parents, and it traumatised him really bad, which is one of the reasons him and his brother don't get along that well.
What Seunghyun is referring to happened quite a few chapters back in a flashback (chapter 48), when he heard his brother say that he didn't have the potential for academics. He basically said this to their parents so they wouldn't force Seunghyun to do something he didn't want, as they were already not there for Seunghyun anyway. But, Seunghyun didn't have context and just assumed his brother, his closest person in the world, didn't believe in him. This is why they drifted apart!
Sorry for the long response!
Way better response than mine tbh, and I personally don't think it's long. Thanks for the explanation
Wait i don't understand. Did i skip a chapter ? What is he talking about