mono = both speakers play the exact same thing at all times
stereo = the music in each speaker varies from right to left (normally to match the original recording)
which is better? For subs mono is better so they don't cancel each other out, and they fall in the freqs your ears don't typicaly notice were they are comming from. For mids and highs, you want them stereo since those are the freqs your ears can tell were they are comming from.