The isobar filters are noise filters on in the device which can dramatically reduce noise on the power line feeding the device plugged into it. For example, an AC motor (like a room fan, refrigerator pump, or hair dryer) or even a Flourescent or LED lightbulb can often generate noise which is fed back into the AC main they are plugged into. Sometimes, under the right conditions, that AC line noise can be heard in the audio from a receiver or other component or be seen as video noise on a TV. By using a power strip with a good noise filter, such as the most effective IsoBar products, that noise is almost completely filtered out and never reaches your sensitive HT gear.
I have been able to measure the residual noise dropping after placing an IsoBar device in my rig.