"Lightweight polyurethane" refers to a type of material that is made from a substance called polyurethane. It is called "lightweight" because it is not very heavy.
Full definition
Carbon - fibre reinforced plastic (CFRP) for front lid, front wings, rear lid and rear wing,
lightweight polyurethane for front and rear aprons, lightweight glass for rear screen and rear side windows, magnesium roof, titanium rear silencer.
The GT2 RS's magnesium roof and thinner rear glass are used and the bumpers and wheelarches are
lightweight polyurethane.
Today, it is achieved by the purposeful use of
lightweight polyurethane, aluminium and magnesium.
Like on the 2018 911 GT3, the front and rear fascia are made of
lightweight polyurethane.