dBFS
When talking about decibel levels, it's helpful to know what the reference is.
When working with digital audio files, levels are expressed as dBFS. The "FS" stands for full scale. In other words how does the volume of the signal compare to digital zero, the highest usable level.
In general, for audio-only projects, like materials destined for CDs, mp3s, or radio broadcast, the peak levels approach and often reach digital zero. The peak levels for this material would then be 0 dBFS, essentially, as high a volume as possible.
Audio to be used for television, movies, and DVDs are often recorded at a more conservative level. For example, most broadcasters require the peak levels to be no higher than -10 dBFS, 10 dB lower than full scale.