0 a low bed, often on wheels, that is stored under an ordinary bed ready for use by visitors
1 a low bed on wheels that is stored under an ordinary bed and pulled out when it is needed:
We bought a trundle bed for unexpected guests.
Some loft beds even have stowable/trundle beds while retaining the capability to contain workstations and drawers.
Trundle beds are a space-saving idea and allow for an extra separate bed to be available when necessary, but do not require the second space constantly.
