Coaxial cables carry electronic signals over a single conductor surrounded by a shield. The shield keeps the signal from "leaking" out of the cable, and prevents unwanted signals from entering and interfering with the main signal.
The effectiveness of coaxial cable shielding depends on the frequency of the signal. The higher the frequency, the shorter the wavelength of the signal, and the more likely the signal will "leak."
The most common type of shield is made of braided copper, but higher frequencies can leak through the spaces in the braids. Some cables use foil, but the most effective shield is the solid corrugated copper used in industrial cables.
To be effective, the shield of a coaxial cable must be grounded at each end. In most cases, the connector completes the connection to ground, but if for any reason that connection breaks, the shield will not be as effective.