What purpose does the small cylindrical module near the end of my laptop charger cable serve?

• They are called ferrite beads/EMI filters/ferrite chokes and contain the semi-magnetic substance Ferrite made from iron oxide (rust) alloyed with other metals .
• Usually seen on power chords, laptop charging cables and other electronic devices such as  printers and medical equipment.
• Any conductive cable acts as an antenna – if the device produces radio frequency energy, this can be transmittedthrough the cable, which acts as an unintentional radiator.
• What do the beads do? They prevent the cable from sending/recieveing signals.
• It has the property of eliminating the broadcast signals. Essentially, it “chokes” the RFI (Radio Frequency Interfearence) transmission at that point on the cable — this is why you find the beads at the ends of the cables. Instead of traveling down the cable and transmitting, the RFI signals turn into heatin the bead.

What it looks like inside.

Ferrite beads are one of the simplest and least expensive types of interference filters to install on preexisting electronic cabling. For a simple ferrite ring, the wire is simply wrapped around the core through the center, typically five or seven times.