Wikipedia
Braid-breaker
A braid-breaker is a filter that prevents television interference (TVI). In many cases of TVI, caused by a high field strength of a nearby high frequency (HF) transmitter, the aerial down lead plugged into the back of the TV acts as a longwire antenna or as a simple vertical element. The radio frequency (RF) current flowing through the tuner of the TV tends to generate harmonics which then spoil the viewing.
The braid breaker works by preventing RF signals picked up on the outside (common mode signals) flowing into the TV set, while passing RF inside the coax (non common mode) from the antenna.