Recording sound is like the story of the three bears: If the levels (as measured by a VU meter) are too high--that's bad. If the levels are too low--that's bad too. Instead, the levels need to be "Just Right." And the only way to tell if the levels are just right is to watch a VU meter.
So here's the important part: when recording audio, you want to make sure the needle AVERAGES straight up. It will bounce a little below and a little above--that's OK. But on average, it should be pointing straight up.
How are audio levels controlled--to get the desired reading on the VU meter? Nearly every professional recording device has a knob of some sort to increase or decrease the level of the audio being recorded. (Many consumer camcorders do not)
The trick here is to NOT trust your ears. It's easy to listen to an audio source and conclude it "sounds OK." and then ignore the VU meters. This is a big mistake.
This animation (it has no sound, just watch the meter) illustrates GOOD audio levels. The needle AVERAGES straight up. It's OK to dip into the red briefly--and it's OK to drop back to zero. But the average should be straight up.