My ignorance: I had to find a definition for MDF: It means Medium Density Fiberboard
from Home Depot: "Medium Density Fiberboard (MDF)
MDF is made by breaking softwoods down into individual fibers, which are then glued and pressed together. MDF is denser than particle board and has a smooth finish that takes paint very well, making it a great choice for interior projects. MDF can also be used for built-ins, cabinets, raised panels, or simple furniture."