Coaxial cables are used to transport radio frequency signals transmitted from an antenna to its receivers (TV in this case). It is called coaxial since it has two conductors on a single axis.
Coaxial cables are characterized by having a great protection and isolation system, so that their transmission of signals will be clean and without traces of the action of external agents to the cable, which is why it is used, among other functions, for signals of TV.
As coaxial cable is a type of transmission line used to carry these high-frequency electrical signals with low losses, the impedance of the cable must match that of the system sending the signal and that of the receiving system, in order to that there are no transmission losses, being 75 ohms the standard that has been used historically in all video systems. For this reason all these cables have a characteristic impedance of 75 ohms.