In a practical sense of the word for car audio, it allows you to have different wiring options. When you wire up your subs to your amp, you can wire it in different ohm "loads".
So look and see how many watts your subwoofer puts out at each ohm level. The power your amp can put out usually increases as you decrease the ohm load, but be careful because some amps aren't recommended to go below 2 ohms because of overheating issues and frying internal structres in the amp.
Voltage (volts) = Current (amps) x Impedance (ohms)
One ohm is the resistance value through which one volt will maintain a current of one ampere.
Now with power, Power (Watts) = Voltage (volts) x I (current)