Ferrite rings
Ferrite rings are components made of magnetic ferrite and are used to filter interference frequencies.
They are placed around cables to attenuate so-called common mode interference: When a ferrite is placed around a coaxial cable, an increased inductance is created at this point. Together with the capacitance already present in cables, a ferrite represents a PI low-pass filter. It therefore filters out high-frequency interference from the transmitted signal.
Further detailed information on ferrite rings can be found here: EMC shielding of cables and lines..