It takes its starting point of O dB as the sound pressure of the most quiet sound that can normally be heared. This is called the threshold of hearing. 0 dBA is therefore the minimum sound pressure that we can here. Because everything is a number of dB above a defined level, then any sound pressure at a level of say 100 dBA will have a known value. The intensity of sound that we perceive is proportional to the square of the sound pressure level.
Ears are very remarkable devices.