A trousseau is a wardrobe for a newly married woman. Literally clothes and accessories, so if mom is buying you definitely want one. While the tradition of a trousseau has become outdated, some women have turned the trousseau into their honeymoon wardrobe or lingerie specifically.
Ask mom what she is thinking to find out what type of clothes or lingerie she wants to buy you. This could be a fun outing for the two of you to spend time together before the wedding.