A trundle bed is a bed that is underneath another one. It is as simple as that. They are beds that slide out from underneath so that a guest can sleep in the room as someone else. In current times they are normally used in a child's room so that a friend can sleep comfortably and not on the floor. In the past the main reason was so that a servant could sleep close by in case she was needed. They are on rollers to make the slide easy and are called trundles because of the rollers which were sometimes referred to by the same name.
Trundles are used as space savers so that two beds won't take up too much space. They can come in king, queen, full, or twin size and sometimes even day beds. The trundles are twin size. The platform that rolls out is a metal frame that has springs stretched across to give it some support and comfort. The rollers are on the bottom for ease of movement. A mattress fitting the size of the trundle is placed on top and may have to be a little thinner than a normal mattress so that it can fit underneath the bed.
Some might feel awkward about making a guest sleep close to the floor -- not as much with children, but if you have a queen bed with a trundle or some other variation, and it is intended for adults, then it might seem weird making them sleep lower than others. If this is the case there are trundles that come spring loaded and raise up. When pulled out, a latch is released that is holding it down, and a spring raises it up to the same height as the regular bed. The frame is then latched into place so that is does not lower when slept on. To lower the trundle and put it back under the bed, simply reverse the process and slide it out of sight.