It was in reference to mains power noise... such as washing machines, dryers, power tools etc that may be causing a really bad noise increase, the advice was to NEVER put one on your DSL cable (which seems sensible enough) as it will increase loss/attenuation, but if you have something that's doling out lots of noise, then apparently putting Ferrite Cores on the device in question's mains cable is helpfull..?!