On an amplifier, the ohm is the impedance. The lower the impedance drops from the amplifier, more power will be produced. A car amplifier operates on 12 to 18 volts and it operates on 14.4 volts with the engine running. It will operate on 12 volts with the engine off. It's not a good idea to be running it with the engine off. The battery will get very hot, stress out and die within an hour on a 1,000 watt system. Then you're gonna need to jump start it with another car.
Here's an example to help you understand. The wattage increases as the impedance drop.
9,000 watts at 1 ohm (18 volts)
7,500 watts at 1 ohm (14.4 volts)
4,500 watts at 2 ohm (14.4 volts)
2,000 watts at 4 ohm (14.4 volts)
But it doesn't mean the amplifier will give you that much power, your car's alternator needs to be able to produce it. The alternator is also powering other things in your car when the engine is running. When the engine is off, the car's battery becomes the power source. The battery keeps the car's computer alive monitoring the gas level, the oil level, the car's health, and to start up the car's engine. This is why the battery becomes dead when the car hasn't been driven for a long time.
On a subwoofer, there's a DVC (dual voice coil) and a SVC (single voice coil). The DVC will give you a more wiring option to change the impedance level. Most people commonly go with a DVC. Now, the next thing is the RMS power (root mean square). This is the recommended RMS power that the woofer can be powered with without the voice coil burning out. Ignore the peak power because you don't need to know that.