箱 is a bound morpheme (b.f.) that appears in compound words to mean various types of containers, such as boxes, cases, chests, or trunks. It is not used alone; instead you combine it with another character to form common words like 箱子 (xiāngzi, 'box/case'), 冰箱 (bīngxiāng, 'refrigerator'), or 信箱 (xìnxiāng, 'mailbox').