what do they mean..
which one should i look at .

 

engine off...engine on...

 

a battery puts out about 12V with the engine off.
when the engine is running, the alternator puts out about 14V, in order to charge the battery and power the electrical loads in the car.

As for looking at amp specs, you need to look at both... a large difference in power output shows either a poorly regulated or an unregulated power supply. Some amps have unregulated supplies so that you can dump lots of voltage in them for short amounts of time and get ridiculous amounts of power.

i.e. Soundstream and Orion HCCA series..
the old soundstream Class A series used to put out about 50x2 at 12v and 75x2 at 14V. you could run them at 16V for short amounts of time and get over twice the rated power out of them..
Guy in SPL competition used these very often.. the Class A amps would also run just about any load you put on it.. many guys had them wired down to 1/2 or 1 ohm mono, so they'd have two amps driving a van full of subs... then just install a couple 8 or 9 V batteries in series instead of a single 12V, and you'd have tons of power and tons of SPL.

 

an unregulated amp will actually take advantage of the 14+ volts of power......and will make more wattage.....a regulated one will only use 12 volts..and will make the same power with the car on or off..