noun
neckband lining; inside collar
Refers to the inner lining of a collar, especially on traditional Japanese clothing like kimono.
着物の裏襟が汚れたので、取り替えてもらった。
The neckband lining of my kimono got dirty, so I had it replaced.
襟 is the collar itself; 裏襟 is specifically the lining on the inside of the collar.
Compound of 裏 (ura, 'back, inside') and 襟 (eri, 'collar').