Bifold sofa means it is made in two parts and will seat 3 and sleep 2 people. Constructed from a hard wood such as Oak or Steel.
Steel Bifold Frames
The better steel frames have "flexi slats" that are more comfortable than rigid Ash or Pine slats. Poor quality steel frames usually have a mesh or bars for the sitting sleeping "decks". Steel bifold frames use clic-clac hinges that are easy to use fro the front of the frame.
Mattress choice depends on filling and personal preference for firmness. Inexpensive bifold mattresses will use cotton that generally compacts and is quite heavy. However futon mattresses filled with a combination of wool, coir, latex etc are considered superior.