What is the bold part on monitor and some charger cables
It is called a common mode choke.  It is a toroidal piece of ferrite with the wires of the cable passing thru the hole.

The cable and the ferrite form a small-valued inductor, but only to the sum of the currents in the cable.  This inductor increases the impedance at high frequencies.  The purpose is usually to attenuate these frequencies so that RF noise created in the monitor doesn't get onto the cable.  The cable can act as a antenna and radiate the signal, and conduct it into whatever the other end of the cable is plugged into.

This choke works both ways.  It also attenuates high frequency noise coming into the monitor.

Note that this affect only works on the total sum of the currents in the cable.  Differential signals that are properly balanced have the same current going out on one wire as coming in on another.  The total current going thru the hole in the ferrite as a result of such a differential signal is therefore zero.  This means no magnetic field is created, and the ferrite presents no increase of impedance to the differential signal.

Unintended noise signals coming out of the monitor are mostly common mode, since they are superimposed on all the conductors of the cable.  These do cause a net current to be seen by the ferrite, which causes a magnetic field, which increases the impedance to those signals.  Currents all going in one direction are called common mode, which is why this is called a common mode choke.