dB is short for decibel, which is a unit that indicates ratio or gain. It is often used to measure sound intensity, signal strength, attenuation and other quantities.
For example, if a sound has a power of 10 W and another sound has a power of 1 W, then the difference in decibels is 10 dB = 10 log (10/1) = 10 log 10 = 10.
Signal Noise Ratio is also measured by dB