When amps are rated as being 2 ohms stable this means that they can withstand the stress put on it by bridging channels and/or hooking up speakers to it in parallel. Each time you reduce ohms to generate more power you exponentially increase the load on the amp. When this happens you increase the chances of blowing fuses or burning out your amp and a worse case scenario would be frying your car's electrical system.

When an amp is rated at being 2 ohms stable the manufacturer is saying: "Hey, we tested this baby at 2 ohms and it performed flawlessly without any adverse side effects to the amp or the vehicles electrical system." I've seen amps that were rated as being 2 ohm stable being run at 1 ohm which resulted in a blown amp.