In an electrical circuit operating at signal voltages (usually less than 50 V or so), a common return path that is the zero voltage reference level for the equipment or system. This signal ground may or may not actually be connected to a power ground. A system where the system ground is not actually connected to earth is often referred to as a floating ground.

Will those so-called "grounding kits" actually make a difference in how well your car runs? Surprisingly enough - maybe.

It is, indeed, possible for multiple sensors in a car that're meant to share the same ground to actually be separated from ground by varying amounts of resistance. In AC systems this can get quite complicated, but car power is (for now, at least...) a simple 12VDC setup where the negative terminal is the ground (and isn't really grounded, seeing as the car's separated from the road by its insulative tyres - this is why cars can accumulate so much static electricity as you drive around on a dry day). In DC circuits, there's not going to be any weird dynamic reactance stuff going on; everything's just simple volts, amps and ohms, and you can do the calculations on the back of one envelope.

If a sensor doesn't see as many volts between positive and ground as it ought to, its readings may be off. And sensors that share supply wires can (in theory at least) float each others' ground up and down depending on how much current they're passing. Solidly link all of the ground points together with nice straightforward cable, rather than relying on possibly corroded, possibly painted, possibly heavily lubricated, possibly loosely connected structural members and mechanical components, and you remove such awkward variables.

Whether there's actually any need to do this, though, can be quite easily determined. Take your faithful multimeter and see if, while the car's running, there's a potential difference between the locations the Ground Net instructions tell you to hook together. If the meter settles to something very close to 0.000 volts, as it will if you touch the probes to two ends of a good conductor that's not passing much current, then there's no need for extra grounding.

If you want to fiddle some more, then try the resistance range, with or without the car running; if the resistance between the points-to-be-grounded is trivially higher than the fraction of an ohm you read when you just touch the probes together (multimeter leads are not superconductors), then once again, all signs point to the ground-net just being decorative.

Someone will notice a difference from any voodoo car modification, computer tweak or medical treatment, so the fact that there are people in the thread you linked to reporting noticeable results from the Ground Net does not surprise me. Note that there's also someone reporting that his lights now don't dim when he starts his car, despite the fact that I don't think this mod has anything to do with the electron path to or from the lights or the starter motor.