The sheet plastic is heated until it becomes soft. The table with your mould on is lifted into position and a vacuum is used to draw the plastic over the mould. Vacuum forming only works with thin plastics and moulds with no undercuts. The plastic can then be trimmed to the required shape.
This technique is widely used for forming thermoplastic materials. It is similar to the casting technique used to form metal components. The plastic (in a powdered or granular form) is put in a hopper. A screw thread turns forcing the plastic material through a heater, melting it. When all the materials has melted the screw thread then acts as a ram and forces the plastic into mould, where it cools and solidifies.