第二ボタン (daini botann) refers to the second button from the top on a shirt or a jacket. In Japan, a graduating boy gives this second button on their school uniform to a girl he likes. This usually occurs at or immediately after a high school graduation ceremony, when the boy threads the button out of his shirt and gives it to a (usually younger) girl, for whom the gift of this button is considered a tremendous honor.

Interesting ritual I found in a manga one shot today.

because: https://dynasty-scans.com/chapters/i_picked_up_a_bunny_girl