When you're recording audio in the field, it usually comes in one of two flavors - mic level or line level. If you don't deal with each one correctly you can end up with audio that is either very distorted or audio you can barely hear.
Mic level audio is exactly what it sounds like... it's audio directly from the microphone with no amplifiers in between the microphone and your camera. When you plug a microphone into your camera's mic jack this is the flavor of audio you're getting. So normally there's no problem with this type of audio.
Line level audio is audio that's been amplified. Typical sources of this kind of audio are feeds from public address systems or microphone mixers. You'll run into line level audio if you are plugged into a audio feed at an event. If you feed line level audio directly into your camera's mic jack you'll end up with distortion and your audio will sound like it was recorded next to a jet engine. If you have an audio meter on your camera, very often (but not always) the audio level will be pushed all the way to the right. After audio is recorded like this, there is no way to fix it.
This is why I use a microphone adapter box from Beachtek. It has a mic/line switch. When I have a line level feed I flip the switch to "line". When I'm just plugging in my microphone directly I switch it to "mic".
This works well unless I leave the switch in the wrong position. If I plug a microphone directly into the adaptor box and the switch is set to "line", I'll get audio I can barely hear. I can boost the level of this audio when I edit it, but it's likely to have a lot of hiss or other noise in it, making it hard to make out what the speaker is saying.
This is why I always carry a good pair of headphones and wear them while I'm recording. Not only can I hear that the audio isn't distorted or too low, but I can also hear if there are any other problems with the audio such as buzzes from cell phones or shorts. More on those types of problems and how to solve them in another post.