A sheet of plastic material is heated to pliability, then pressed against a 3D mold by vacuuming out the air between the sheet, and the mold.
In other words, Vacuum Forming is where a sheet of plastic is heated to a forming temperature, stretched onto a single-surface mold and forced against the mold by a vacuum. This process can be used to form plastic into permanent objects such as turnpike signs and protective covers.