A secondary suite is basically a home within a home. It's a section of a structure (and occasionally a separate related structure) for residents -- sometimes known as a mother-in-law's suite, granny suite or guesthouse. Sometimes it's just a separate bed and bathroom; other times it may also have cooking space (a kitchenette or even a small, full kitchen) and a social area (like a living room or den). As well as being used for guests or relatives who visit or move it, it can be used as a rental or for older children who haven't moved out, but may desire a bit more independence and privacy.
Compact furnishings or furniture that can serve multiple purposes can be very useful in a secondary suite. Also furnishings, such as folding tables, that can be "put away" when not in use provide a bit more flexibility in the use of the, often limited, space.
- Wikipedia's article on secondary suite