Ferrite ring reduces high-frequency noise

The ferrite ring acts as a common-mode, inductive filter of the high-frequency noise produced by the switching of the MOSFET’s inside of the ESC. Basically, it prevents the AC current fluctuations inherent in MOSFET switching from progressing beyond the ring itself (it actually looks like a big resistor to high-frequency currents and doesn’t affect DC signals at all), thus eliminating the potential for making your ESC signal wire an antenna that radiates at the frequency of the common-mode noise. It also prevents that noise from getting into your receiver or servo signal wires.