MacBook Pro UNIBODY models have a protective glass panel overlaying the actual LED screen. This is useful because If the picture is still visible and the screen working correctly following damage to the display - a crack, break, scratch for example - then the front glass is all that needs to be replaced.
Oddly Apple does NOT sell replacement glass panels requiring instead that the entire top 'half' of the computer be replaced - an expensive waste of resources. Thankfully third party manufacturer's (OEMs) have stepped in to fill the gap by offering two inexpensive solutions, one made with tempered glass, the other from a plexiglass composite.
Tempered glass panels are more expensive but they match the original parts used by Apple. Plexiglass panels save you money and unless you are a purist are not terribly different from the original glass versions. We offer both options. The main point being that in both cases they provide a sensible alternative to buying an expensive full display assembly when a replacement front panel is all that's needed.