The word coaxial means being mounted on the same axis or having the same radial axis. For a coaxial cable this means that one conductor surrounds the other.
A coaxial cable consists of a central conductor surrounded by insulation. It is then surrounded by a copper mesh, and finally an outer layer of insulation.
The central conductor, which is usually made of copper, carries the signal. The copper mesh provides the return path and also shields the central conductor from picking up noise.
The speed at which the signal travels in the cable is determined by the dielectric constant of the insulation material between the two conductors. For the common RG58 cable the signal travels at 0.66 times the speed of light.
Coaxial cables are often used for RF signals, for example in cable TV networks. Previously it was used for long-distance telephone networks and for computer networks, but is often replaced by fiber optic cables (both telephone and computer networks) and twisted-pair cables (computer networks only).