A boxroom is a small room often found in older houses in the UK, traditionally used for storage -- i.e., a room full of trunks or boxes. Boxrooms might house unused hatboxes, dress boxes, unused luggage, furniture and other large items.
In more recent years, boxrooms have taken on additional uses, most often as a small bedroom for children, live-in servants, or guests. It is now common to refer to a small bedroom as a boxroom, and to have them set up as a bedroom, even if they may also be used for storage when not in use.
Boxroom is also written as box-room or box room. Wealthier households might instead have a lumber room, which is the same basic idea, but also stored unused furniture.