1. dB means Decibel  which is 1/10th of Bel.

2. Decibel (dB), unit for expressing the ratio between two physical quantities, usually amounts of acoustic or electric power, or for measuring the relative loudness of sounds. One decibel (0.1 bel) equals 10 times the common logarithm of the power ratio.

3.We need dB calculation because decibels Can Represent Large Ratios With Simple Numbers.Decibels are logarithmic. They allow simple numbers to express values that would normally require many more digits.

4. It allows to calculate different types of parameter's value in an identical unit.

5. Some short-cuts for dB 

20 dbW= 20+30 dBm=50 dBm

Always add 30 for dBW to dBm

30 dBm = 0 dBW

100 dBm = 100- 30 dBW =70 dBW

Always subtract 30 for dBm to dBW 

dBW + dBW = dBW

dBW - dBW = dB

dBW + dB = dBW

dbW - dB =  dBW

dBm + dBm = dBm

dBm - dBm = dB

dBM + dB = dBM

dbM - dB =  dBM