What is meant by the thickness to chord ratio of an aerofoil?

Chord ratio:

It is the ratio of thickness to the chord. It is also knows as thickness ratio.

Chord ratio measure the performance of wing when it is operating at the transonic speed.

When the speed cross the speed of sound wave then that wave creates the shock wave and these shock leads to produce drag force on the aerofoil profile. When Mach number is one then it means that if Mach increase then it will leads increase then drag force.