Tempered glass is also called pre-stressed glass. It is made by heating the glass and then cooling it very quickly. This type of glass breaks differently than normal glass, creating more rounded edges of glass pieces instead of jagged shards that regular glass creates.
A tempered glass protector is a multi-layered screen protector. It has a bottom layer of absorbent silicon, PET film and an optically clear adhesive tempered glass and oleophobic coating in that order. Tempered glass is up to five times stronger than normal glass.
Tempered glass is more heat resistant than regular glass, meaning your phone is less likely to be damaged by heat.
The smoothness and clarity of the tempered glass makes the screen protector feel more like your actual smart phone screen than a plastic screen cover would. Generally, tempered glass screen protectors are generally easier to put on your phone than plastic screen protectors.