A trundle bed is a bed that stores itself under another bed, usually on rolling casters or wheels. In the US, it is called a trundle bed, while in the UK, it is referred to as a truckle bed.
In hotels and other places of accommodation, a trundle bed is a kind of space-saving furniture, designed to easily add more sleeping arrangements when necessary.
Trundle beds are often found in smaller hotel rooms or in cramped transport accommodations, such as in train suites (couchette) or cruise ship cabins. It is different than a Murphy bed, which is attached to a wall.
For example, a small cabin may have a twin-sized (single) bed, under which a truckle bed is stored. When more sleeping arrangements are required, one can just pull out the trundle bed.
A trundle bed may be on a spring, so that it can be raised up to the same height as the bed it’s nested under, or it can just be essentially a mattress on wheels and remain that floor height.