That would be HDF, High Density Fiberboard. It is in common use in framing in the UK and Europe, though it is not considered preservation grade in the US. There have been words across the pond about this. HDF is commonly available in the US in thicker panels that are veneered and used in cabinets. i'm really not sure where to send you for panels thin enough to use as backers for art prints.
There are versions of it that are considered preservation grade, but you won't find them at Home Shelter Stores, or the normal line of suppliers to the framing industry. They are also relatively expensive. You might be able to find them through distributors of archival materials like Talas or Gainesboro.
Normal HDF is made from the sawdust and ground wood leftovers of millwork (so it has wood protein in it and develops acidic gasses) and is held together with glues that probably have some kind of chemical out-gassing (peroxide, formaldehyde, etc) that aren't good around most art either.
If preservation is an goal with the art, I would stick with more traditional materials like 100% cotton rag board for both mat and mount. There are other options which are almost as good using materials made from Alpha-cellulose.
