A decibel (dB) is a unit invented to measure noise.  It is a logarithmic unit that expresses the ratio of noise pressure to the noise pressure of the threshold of human hearing.  As it is a logarithmic unit, small increases in the numbers mean large increases in noise just like that other common logarithmic unit the Richter Scale for earthquakes.

An increase of 3dB is double the noise, an increase of 10dB is 10 times the noise.

For reference,

• a level of 30dB is often considered to be dead quiet.
• normal conversation is around 60dB
• A lawn mower produces around 80dB
• A rock concert or a thunderclap is around 115 db

Anything above 85dB is considered to be harmful to hearing.