They are blocks of polyurethane foam or foam rubber of different densities and they are covered with a cloth cover, usually cotton.
Polyurethane is the most common type of foam that can be found in mattresses, and in its medium or firm versions can be used as a support layer perfectly. Unlike its cousin, the viscoelastic (known in English as memory foam, memory foam), is made to compress under pressure. This gives it a greater resilience (ability to recover its shape when the force that causes that deformation ceases) and progressive resistance than the flexible polyurethane foam (or viscoelastic) and can support the heavier parts of the body in a more efficient way. This is the reason why it can be used as a support layer. It is divided into three different types. The lower one has less resilience and less progressive resistance than the higher ones. All types can be made in a greater or lesser firmness. Softness or firmness is not an indicator of quality. For this, we look more at the density or kilos per cubic meter. Below are the different types we are talking about.
Conventional polyethylene foam mattress
This is the lowest grade of polyethylene foam and its weight is less than 24 kilos per cubic meter. It is the cheapest and really is not suitable for your mattress unless we cannot buy something better or for occasional use (we must take regularly with that person). It is also not recommended as a support layer since they do not last long. We can call them soft or soft-medium foam. There is also king size memory foam mattress.
High-density foam mattress
It is also often seen in those labeled as HD, by its English name, High Density. This type of foam goes from 24 kilos per cubic meter to 32, even 40 kilos per cubic meter in some manufacturers. So they range from medium foam range to firm in the case of 32 and extra firm reaching 40. This type of mattress is already more suitable for normal use. Although foam mattresses are no longer a good first choice as a regular mattress. If that is suitable as a top layer of support in mattresses of another type, already of better quality.