TIR is total internal reflection
When light travels from denser medium to rarer medium it moves away from the normal.Therefore angle of refraction is greater than angle of incidence and as the angle of incidence increases even the angle of refraction increases . At one point when the angle of incidence is high the angle of refraction would be 90 degrees and at this point the refracted ray would go along the line of separation.At this point angle of incidence is called the critical angle.When the angle of incidence is even higher than critical angle the refracted ray would go back to the denser medium i.e gets reflected. This phenomenon is called total internal reflection.
The shining nature of the diamond is due to the phenomenon of tir i.e it is cut in such a way that tir takes place

Hope it helped u ;)
When light travels form denser medium to rarer medium rays are not able to penetrate the boundary and come back to same medium(i.e.,denser medium) this phenomenon is known as total internal reflection(simple way)