Coaxial cable is a two conductor cylindrical transmission line  typically comprised of a inner conductor and the outer conductor with an  insulating dielectric material between them. the primary function of coaxial cable is to transmit the RF energy with high performance/ efficiency due to its tight control over its electrical impedance and superior Electromagnetic Interference (EMI) shielding.
There are primarily two impedance values of coaxial cable which are current;y used in industry. First, 50 Ohms impedance which is mainly used in data signal applications and the second is 75 Ohms that is used in video signal applications.