Optical fibres consist of a core of higher refractive index surrounded by a cladding of lower refractive index.

When light travelling through the core strikes the core-cladding boundary at an angle greater than the critical angle, it undergoes Total Internal Reflection and is reflected back into the core instead of escaping.

This repeated Total Internal Reflection allows light signals carrying data to travel long distances through the fibre with minimal loss, enabling high-speed data transmission.

Hence, Total Internal Reflection is the phenomenon that enables data transmission through optical fibres.