Levels
Too Low
The sound is lousy on these two clips, but that's ON PURPOSE! On the left clip, listen to the distortion caused by levels that are recorded too high. On the right clip, you'll hear the hiss that results from levels that are recorded too low.
Again, these problems are not always evident when the sound is first recorded. The hiss and distortion won't become evident until later, when the sound is played back. That's why it is important to watch the levels when recording.
Too High