Baekhyun was wearing a slightly suggestive t-shirt which shows an image of two women lying together with their legs intertwined as a part of his stage costume. During the performance, the image was covered by a jacket that Baekhyun was wearing on top, and he didn't notice what was actually on his shirt until he removed the outer layer.
Looking down and seeing the suggestive image on his shirt, Baekhyun proceeds to cover it with his jacket right away, a look of embarrassment creeping slowly (or is it an impish grin?) up his face.
All I can say is..."Kkaebsong."