They are ferrite beads used to suppress parasitic electrical noise that might interfere with the desired signal.
They protect the system from noise generated externally to the protected system and dampen the noise generated by the protected system so that it does not interfere with other systems. See: Ferrite beads - Wikipedia
Ferrite beads can be made in different compositions to target specific interference frequency ranges.
They're not always gone these days, but cheaper cables usually don't have them. Cheaper cables sometimes rely on shielding or other less effective forms of interference suppression.
I often snap extra beads around the cables and use extra suppressors around the house to further eliminate stray electrical noise so it doesn't interfere with my ham radio reception or my old school, noise sensitive home lighting controls system.
If they claim my ham radio is interfering with their analog TV or radio reception, I'll pass it on to neighbors too. Their cheap or defective receiver is likely the problem, but it's easier to give them ferrite than to argue with them.
I never install them myself because snapping them on is a trivial task, and I don't want them to blame me for something else they imagined. Ferrites always solve the problem and keep the good neighbors.
When we had an analogue TV in our own home, the ferrite snapping on its cable greatly contributed to the tranquility of the home. Digital TV solves this problem, but ferrite still has to go online. Ferrites can sometimes confuse cable TV repairers.
I once got permission from a neighbor to snap 100 ferrites around the wires connecting the low voltage lighting system buried in the ground in his garden after the noise from his cheap switching power supply completely blocked my radio signal every night.
The ferrite didn't work, so with his permission I paid for a new, higher quality linear power supply for his system and had an electrician install it.
I live very close to other people in the city, and the brutal electronic environment is very noisy. However, the system in my own home is now very quiet, thanks in large part to the ferrite beads.
