Tempered glass is “safety glass”. It is normal glass (annealed) that has been heat treated to make it stronger. It is required in all doors and in certain windows depending on their location. Tempered glass is difficult to break but when it does break it breaks into many small pieces instead of large dangerous pieces. Hopefully this will prevent serious injury in the event that someone walks through your patio door. Building codes determine where tempered glass is required and your contractor will determine those locations.