What is MDF? MDF stands for Medium Density Fibreboard which is a type of composite wood product that constructed by taking little wood fibers and sticking them together with resin under extreme heat and pressure. The reason that we utilizing this sort of material is MDF board is stable. It does not expand as well as contract in moist or humid environments, which would normally cause the cracking problem. Therefore, as we use the MDF board for the back cover of our frame, it does not damage easily.