Don't you just hate it when a commercial suddenly interrupts your favorite television show, especially when its volume is twice as loud as the show you were just watching? Well, in audio recordings, the same thing can happen. If you're interviewing someone on the phone through Skype, or turn your head just slightly and continue talking, there can be significant variations in your volume levels, and consequently your audio will have different volume levels. But this is fixable. You can use the Audacity Compressor utility to make your volume more uniform across the entire track.
The compressor tool lowers the volume of the louder portions of the audio track, and leaves the softer parts alone. This process is called dynamic range compression.