subs = really loud
highs = not as loud
to make your music sound good all around he is saying that you will need to work on getting your Highs (vocals) loud, which means putting lots of power on the highs, and get the placement of the highs in the right area to make them sound well. As far as the lows, you can choose a solid well built woofer and get a custom box built and tune the box low, so that the sub will play lower= full sound in a car= loud highs and solid lows.