Answer: Ferrite beads
The clumsy hard plastic cylinder at the end of many cables that you find around your home and office USB cables, video cables and so on-have a specific feature. Inside the hard plastic case is a piece of ferrite-based ceramic designed to suppress high-frequency sound. In essence, the ferrite bead seems like a passive low pass filter that both blocks energy from "outside" sources that can interfere with the cable's function and absorb energy that the cable can emit that can interfere with nearby devices. The majority of energy is absorbed by the bead and then released as small amounts of heat in ambient air.
Why is it important to suppress high-frequency sound? Noise reduction serves two primary purposes. First, it prevents electrical noise generated in a device from interfering with interference from other components, so that interference from a computer does not affect display performance. Secondly, it prevents electrical and radio energy from leaking and interfering with interfering equipment such as televisions and radios. Ferrite beads are one of the simplest (and definitely one of the cheapest) types of interference filters installed on existing cabling, if needed, because you can buy cheap snap-on models that clog over existing cables.
Image courtesy of B & H.