A zolder is an attic. You may come across the term in estate inventories, that describe the items per room. People often used zolders for storage.
In some cases, the zolder would be used to sleep. Attics were often poorly insulated, so this could be cold unless you slept next to the chimney. I heard a story of a Frisian family where the older boys had to sleep in the attic over a cow shed, which was very cold in winter. The house was not big enough for all the children so only the younger children got to sleep in the house.