I bet no one ever thought of this until now
So you have been using your laptops for years
And it is mostly plugged to an electric socket
Now people are wondering, what small cylinder?
Yes that small Cylinder
The cylinder is known as Ferrite Bead.
It isolates the laptop from the electro-magnetic noise that occurs from the waves picked up by the cable, or from the noise in AC-DC converter.
It dissipates high-frequency currents in a ferrite ceramic, and helps make the most powerful noise suppression device! It is called a core, block, or EMI ring.
So, that is what it is all about.
It is a kind of electronic choke