It is a measure init of acoustic sound pressure. Acoustic sound pressure levels (SPL’s) measured in dB are commonly referenced to a level of 20×10-6 Pa (Pascal) o 20 µPa. This is the sound level of the threshold of human hearing. The SPL in decibels is the number of decibels above this threshold.
A conversation at 1 m distance -> 60 dBSPL, Rock concert -> 110 dBSPL and jet takeoff at 450 m -> 120 dBSPL.
The minimum threshold of human hearing is in 0 dBSPL and maximum threshold of pain level is in 130-140 dBSPL.